Until the new functionality becomes available you can use the workaround solution in the attached project.

Excel does not support dates prior to 1/1/1900, however,  we could export such dates as simple strings: https://support.office.com/en-us/article/excel-specifications-and-limits-1672b34d-7043-467e-8e27-269d656771c3

How to reproduce:
 public partial class Form1 : RadForm
 {
     public Form1()
     {
         InitializeComponent();

         this.radGridView1.DataSource = this.GetData();
         this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
     }

     private object GetData()
     {
         DataTable dt = new DataTable();
         dt.Columns.Add("Id", typeof(int));
         dt.Columns.Add("Name", typeof(string));
         dt.Columns.Add("Bool", typeof(bool));
         dt.Columns.Add("Date", typeof(DateTime));

         DateTime date = new DateTime(1850, 1, 1);
         for (int i = 0; i < 100; i++)
         {
             dt.Rows.Add(i, "Name " + i, i % 2 == 0, date.AddYears(i));
         }

         return dt;
     }
     
     FieldInfo fi;
     private void radButton1_Click(object sender, EventArgs e)
     {
         //Old Export using the ExcelML format
         this.fi = typeof(Telerik.WinControls.UI.Export.ExcelML.CellElement).GetField("_dataElement", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);
         ExportToExcelML exporter = new ExportToExcelML(this.radGridView1);
         string fileName = @"..\..\data.xls";
         exporter.RunExport(fileName);

         //New export utilizing the SpreadProcessing libraries
         GridViewSpreadExport spreadExporter = new GridViewSpreadExport(this.radGridView1);

         SpreadExportRenderer exportRenderer = new SpreadExportRenderer();
         spreadExporter.RunExport(@"..\..\data.xlsx", exportRenderer);
     }
 }


Workaround: handle the CellFormatting event
public partial class Form1 : RadForm
{
    public Form1()
    {
        InitializeComponent();

        this.radGridView1.DataSource = this.GetData();
        this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
    }

    private object GetData()
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("Id", typeof(int));
        dt.Columns.Add("Name", typeof(string));
        dt.Columns.Add("Bool", typeof(bool));
        dt.Columns.Add("Date", typeof(DateTime));

        DateTime date = new DateTime(1850, 1, 1);
        for (int i = 0; i < 100; i++)
        {
            dt.Rows.Add(i, "Name " + i, i % 2 == 0, date.AddYears(i));
        }

        return dt;
    }
    
    FieldInfo fi;
    private void radButton1_Click(object sender, EventArgs e)
    {
        //Old Export using the ExcelML format
        this.fi = typeof(Telerik.WinControls.UI.Export.ExcelML.CellElement).GetField("_dataElement", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);
        ExportToExcelML exporter = new ExportToExcelML(this.radGridView1);
        exporter.ExcelCellFormatting += Exporter_ExcelCellFormatting;
        string fileName = @"..\..\data.xls";
        exporter.RunExport(fileName);

        //New export utilizing the SpreadProcessing libraries
        GridViewSpreadExport spreadExporter = new GridViewSpreadExport(this.radGridView1);
         spreadExporter.CellFormatting += SpreadExporter_CellFormatting;

        SpreadExportRenderer exportRenderer = new SpreadExportRenderer();
        spreadExporter.RunExport(@"..\..\data.xlsx", exportRenderer);
    }

    private void SpreadExporter_CellFormatting(object sender, Telerik.WinControls.Export.CellFormattingEventArgs e)
    {
        if (e.GridRowIndex >= 1 && e.GridCellInfo.ColumnInfo is GridViewDateTimeColumn && ((DateTime)e.GridCellInfo.Value).Year < 1900)
        {
            Telerik.Windows.Documents.Spreadsheet.Model.CellSelection cell = e.CellSelection as Telerik.Windows.Documents.Spreadsheet.Model.CellSelection;
            cell.SetValue(e.GridCellInfo.Value.ToString());
        }
    }

    private void Exporter_ExcelCellFormatting(object sender, Telerik.WinControls.UI.Export.ExcelML.ExcelCellFormattingEventArgs e)
    {
        if (e.GridRowIndex > -1 && e.GridCellInfo.ColumnInfo is GridViewDateTimeColumn && ((DateTime)e.GridCellInfo.Value).Year < 1900)
        {
            DataElement data = new DataElement();
            data.DataItem = e.GridCellInfo.Value.ToString();
            this.fi.SetValue(e.ExcelCellElement, data);
        }
    }
}

Workaround: handle the GridViewPdfExport.CellFormatting event and apply the column`s format string

Public Class Form1
    Sub New()

        InitializeComponent()

        Me.RadGridView1.DataSource = Me.GetData()
        Me.RadGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill

        Dim decimalColumn = DirectCast(Me.RadGridView1.Columns("Money"), GridViewDecimalColumn)
        decimalColumn.DecimalPlaces = 0
        decimalColumn.FormatString = "{0:C0}"

        Dim dateTimeColumn = DirectCast(Me.RadGridView1.Columns("Date"), GridViewDateTimeColumn)
        dateTimeColumn.FormatString = "{0:D}"

    End Sub

    Private Function GetData() As Object
        Dim dataTable As New DataTable()
        dataTable.Columns.Add("Id", GetType(Integer))
        dataTable.Columns.Add("Name", GetType(String))
        dataTable.Columns.Add("Money", GetType(Decimal))
        dataTable.Columns.Add("Date", GetType(DateTime))

        For i As Integer = 0 To 999
            dataTable.Rows.Add(i, "Name " & i, i * 10, DateTime.Now.AddDays(i))
        Next

        Return dataTable
    End Function

    Private Sub RadButton1_Click(sender As Object, e As EventArgs) Handles RadButton1.Click
        Dim pdfExporter = New GridViewPdfExport(Me.RadGridView1)
        AddHandler pdfExporter.CellFormatting, AddressOf pdfExporter_CellFormatting
        Dim renderer = New PdfExportRenderer()
        Dim fileName As String = "..\..\exported-grid.pdf"

        pdfExporter.RunExport(fileName, renderer)
    End Sub

    Private Sub pdfExporter_CellFormatting(sender As Object, e As PdfExportCellFormattingEventArgs)
        If e.RowIndex > -1 Then
            e.CellElement.Text = TryCast(RadDataConverter.Instance.Format(e.Row.Cells(e.ColumnIndex).Value, GetType(String), e.Column), String)
        End If
    End Sub

End Class

To reproduce: if you pin a calculator or a hyperlink column you will notice that the cells belonging to these columns remain white. However, the cells from other column types  have a light blue/gray fill color for pinned state. 

Workaround: 
private void RadGridView1_CellFormatting(object sender, CellFormattingEventArgs e)
{
    if (e.Column.IsPinned)
    {
        e.CellElement.BackColor = Color.FromArgb(235, 244, 252);
    }
    else
    {
        e.CellElement.ResetValue(LightVisualElement.BackColorProperty, ValueResetFlags.Local);
    }
}

To reproduce: please refer to the attached gif file and sample project.

Workaround: when you set the DrawFill property to true, specify the BackColor to the desired one and reset for the rest of the cells.

private void radGridView1_CellFormatting(object sender, Telerik.WinControls.UI.CellFormattingEventArgs e)
{
    if (e.Column.Name == "ProductName" && e.Row is GridViewDataRowInfo)
    {
        if (e.CellElement.Value.ToString().Contains("C"))
        {
            e.CellElement.DrawFill = true;
            e.CellElement.BackColor = Color.Yellow;
            e.CellElement.GradientStyle = GradientStyles.Solid;
            e.CellElement.ForeColor = Color.OliveDrab;
        }
        else
        { 
            e.CellElement.ResetValue(LightVisualElement.DrawFillProperty, ValueResetFlags.Local);
            e.CellElement.ResetValue(LightVisualElement.GradientStyleProperty, ValueResetFlags.Local);
            e.CellElement.ResetValue(LightVisualElement.ForeColorProperty, ValueResetFlags.Local);  
            e.CellElement.ResetValue(LightVisualElement.BackColorProperty, ValueResetFlags.Local);  
        }
    }
    else
    { 
        e.CellElement.ResetValue(LightVisualElement.DrawFillProperty, ValueResetFlags.Local);
        e.CellElement.ResetValue(LightVisualElement.GradientStyleProperty, ValueResetFlags.Local);
        e.CellElement.ResetValue(LightVisualElement.ForeColorProperty, ValueResetFlags.Local);    
        e.CellElement.ResetValue(LightVisualElement.BackColorProperty, ValueResetFlags.Local);  
    }
}

To reproduce:
- Add a RadGridView and a button to a blank form.
- Subscribe to CellValidating event from the grid and set the cancel property of the CellValidatingEventArgs to true upon some condition.
- Add click event handler for the button and print a message in it.
- Start the application and enter some invalid data in the grid cell, then click the button.
- The code from the button's event handler is executed.

Workaround use a flag to determine when to execute the corresponding button code:
bool validating = false;

void radGridView1_CellValidating(object sender, Telerik.WinControls.UI.CellValidatingEventArgs e)
{
    if (e.Value.ToString().Length < 5)
    {
        e.Cancel = true;
        validating = true;
        e.Row.ErrorText = "Validation error!";
    }
    else
    {
        validating = false;
    }
}

private void radButton1_Click(object sender, EventArgs e)
{
    if (!validating)
    {
        Debug.WriteLine("Executed");
    }
}

To reproduce: please refer to the attached screenshot.

Workaround: add the FilterDescriptor programmatically: https://docs.telerik.com/devtools/winforms/gridview/filtering/setting-filters-programmatically-(simple-descriptors)

To reproduce:

            this.radGridView1.Columns.Add("FIRST CELL ID A");

            this.radGridView1.Rows.Add("405-55-214-41763");
            this.radGridView1.Rows.Add("405-55-214-46682");
            this.radGridView1.Rows.Add("405-55-214-46682");

            this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
            this.radGridView1.MultiSelect = true;
            this.radGridView1.SelectionMode = Telerik.WinControls.UI.GridViewSelectionMode.CellSelect;

Please refer to the attached gif file illustrating better the text moving. 

Workaround: 
        private void radGridView1_CellFormatting(object sender, Telerik.WinControls.UI.CellFormattingEventArgs e)
        {
            e.CellElement.BorderBottomWidth = 2;
            e.CellElement.BorderRightWidth = 2;
            e.CellElement.BorderTopWidth = 2;
            e.CellElement.BorderLeftWidth = 2;
        }

To reproduce:
 this.radGridView1.SummaryRowsBottom.Insert(0, summaryRowItem2);
 this.radGridView1.SummaryRowsBottom.Move(2, 0);

Workaround:
Remove all items and add them back with a spesific order. 

How to reproduce: check the attached project and video

Bind the grid to the following DataTable:

            DataTable dt = new DataTable();     
            dt.Columns.Add("X.Y"); 
            dt.Columns.Add("X");   
            
            dt.Rows.Add("Data X.Y", "Data X");

            this.radGridView1.DataSource = dt;

You will notice that the second column "X" is missing.

Note: if you change the order of adding the columns, both columns will be added.

The new mode should allow copying of single cells even when having the SelectionMode set to FullRowSelect. The attached project features a possible workaround.

How to reproduce:
Create a grid with enabled filtering and open the excel-like filter popup of a DateTime column.

Workaround: 
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        this.radGridView1.DataSource = this.GetData();

        this.radGridView1.EnableFiltering = true;
        this.radGridView1.ShowFilteringRow = true;
        this.radGridView1.ShowHeaderCellButtons = true;
        this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

        this.radGridView1.FilterPopupInitialized += RadGridView1_FilterPopupInitialized;
    }

    private void RadGridView1_FilterPopupInitialized(object sender, FilterPopupInitializedEventArgs e)
    {
        RadDateFilterPopup popup = e.FilterPopup as RadDateFilterPopup;
        if (popup != null && popup.Width < 300)
        {
            popup.Width += 100;
            popup.Height += 100;
        }
    }

    private object GetData()
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("Id", typeof(int));
        dt.Columns.Add("Name", typeof(string));
        dt.Columns.Add("Bool", typeof(bool));
        dt.Columns.Add("Date", typeof(DateTime));

        for (int i = 0; i < 100; i++)
        {
            dt.Rows.Add(i, "Name " + i, i % 2 == 0, DateTime.Now.AddDays(1));
        }

        return dt;
    }
}

How to reproduce: 
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        this.radGridView1.DataSource = this.GetData();
        this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
        this.radGridView1.EnableFiltering = true;
        this.radGridView1.ShowHeaderCellButtons = true;

        this.radGridView1.FilterPopupRequired += RadGridView1_FilterPopupRequired;

        GridViewDateTimeColumn date = this.radGridView1.Columns["Date"] as GridViewDateTimeColumn;
        date.DataType = typeof(DateTime);
        date.FilteringMode = GridViewTimeFilteringMode.Date;
    }

    private void RadGridView1_FilterPopupRequired(object sender, Telerik.WinControls.UI.FilterPopupRequiredEventArgs e)
    {
        if (e.Column.GetType().Name == "GridViewDateTimeColumn")
        {
            RadListFilterPopup popup = new RadListFilterPopup(e.Column, true);
            e.FilterPopup = popup;
        }
    }

    private object GetData()
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("Id", typeof(int));
        dt.Columns.Add("Name", typeof(string));
        dt.Columns.Add("Date", typeof(DateTime));
        dt.Columns.Add("Bool", typeof(bool));

        DateTime date = DateTime.Now;
        for (int i = 0; i < 1500; i++)
        {
            dt.Rows.Add(i, "Name " + i, date.AddDays(i), i % 2 == 0);
        }

        return dt;
    }

Workaround: custom RadListFilterPopup

public class MyRadListFilterPopup : RadListFilterPopup
{
    public MyRadListFilterPopup(GridViewDataColumn dataColumn, bool groupedDateValues)
        : base(dataColumn, groupedDateValues)
    { }

    protected override void OnButtonOkClick(EventArgs e)
    {
        FilterOperator filterOperator = FilterOperator.IsEqualTo;

        IRadListFilterElement listFilterElement = typeof(RadListFilterPopup).GetField("listFilterElement", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic).
            GetValue(this) as IRadListFilterElement;

        switch (listFilterElement.SelectedMode)
        {
            case ListFilterSelectedMode.All:
                filterOperator = FilterOperator.None;
                break;
            case ListFilterSelectedMode.Null:
                filterOperator = FilterOperator.IsNull;
                break;
            case ListFilterSelectedMode.NotNull:
                filterOperator = FilterOperator.IsNotNull;
                break;
        }

        if (filterOperator != FilterOperator.IsEqualTo)
        {
            SetFilterOperator(filterOperator);
            this.ClosePopup(RadPopupCloseReason.CloseCalled);
        }
        else
        {
            CompositeFilterDescriptor compositeFilterDescriptor = new CompositeFilterDescriptor();
            compositeFilterDescriptor.PropertyName = base.DataColumn.Name;
            compositeFilterDescriptor.LogicalOperator = FilterLogicalOperator.Or;

            foreach (DictionaryEntry entry in listFilterElement.SelectedValues)
            {
                foreach (object value in (ArrayList)entry.Value)
                {
                    FilterDescriptor descriptor;
                    if (base.DataColumn is GridViewDateTimeColumn || base.DataColumn.DataType == typeof(DateTime) ||
                        base.DataColumn.DataType == typeof(DateTime?))
                    {
                        descriptor = new DateFilterDescriptor(base.DataColumn.Name, FilterOperator.IsEqualTo, (DateTime?)value, false);
                    }
                    else
                    {
                        descriptor = new FilterDescriptor(base.DataColumn.Name, FilterOperator.IsEqualTo, value);
                    }
                    compositeFilterDescriptor.FilterDescriptors.Add(descriptor);
                }
            }

            base.FilterDescriptor = compositeFilterDescriptor;
            OnFilterConfirmed();
        }
    }
}

To reproduce:

   Sub New()
         
        InitializeComponent()
        Dim dt As New DataTable
        dt.Columns.Add("Id", GetType(Integer))
        dt.Columns.Add("Name", GetType(String))
        For index = 1 To 1000
            dt.Rows.Add(index, "Item" & index)
        Next
        Me.RadGridView1.DataSource = dt


    End Sub
    Private Sub RadButton1_Click(sender As Object, e As EventArgs) Handles RadButton1.Click
        Dim pdfFile As String = "..\..\exportedFile.pdf"
        Dim pdfExporter As New GridViewPdfExport(Me.RadGridView1)
        pdfExporter.ShowHeaderAndFooter = True
        pdfExporter.FitToPageWidth = True
        AddHandler pdfExporter.PdfExported, AddressOf pdfExporter_PdfExported
        Dim renderer As New PdfExportRenderer()
        pdfExporter.RunExport(pdfFile, renderer)

        Process.Start(pdfFile)
    End Sub

    Private Sub pdfExporter_PdfExported(sender As Object, e As System.EventArgs)

        Dim pdfFile As String = "..\..\exportedFile.pdf"
        Dim document As RadFixedDocument
        Dim provider As Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.PdfFormatProvider = New Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.PdfFormatProvider()
        Using stream As Stream = File.OpenRead(pdfFile)
            document = provider.Import(stream)  '<==== Error Found
            For Each page As RadFixedPage In document.Pages
                Dim editor As FixedContentEditor = New FixedContentEditor(page)
                editor.Position.Translate(page.Size.Width / 2, page.Size.Height - 50)

                Dim pageNum As Integer = document.Pages.IndexOf(page) + 1
                editor.DrawText(pageNum + " of " + document.Pages.Count)
            Next
        End Using

        Using output As Stream = File.OpenWrite(pdfFile)
            provider.Export(document, output)    '<==== Error Found
        End Using

        Process.Start(pdfFile)
    End Sub

Workaround: use the PdfExportRenderer.PdfExporting event where you have access to the document and you can make any customizations to it:
        Dim pdfFile As String = "..\..\exportedFile.pdf"
        Dim pdfExporter As New GridViewPdfExport(Me.RadGridView1)
        pdfExporter.ShowHeaderAndFooter = True
        pdfExporter.FitToPageWidth = True
        Dim renderer As New PdfExportRenderer()
        AddHandler renderer.PdfExporting, AddressOf PdfExporting
        pdfExporter.RunExport(pdfFile, renderer)

    Private Sub PdfExporting(sender As Object, e As PdfExportingEventArgs)
        Dim document As RadFixedDocument = e.Document
        Dim provider As Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.PdfFormatProvider = New Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.PdfFormatProvider()

        For Each page As RadFixedPage In document.Pages
            Dim editor As FixedContentEditor = New FixedContentEditor(page)
            editor.Position.Translate(page.Size.Width / 2, page.Size.Height - 50)

            Dim pageNum As Integer = document.Pages.IndexOf(page) + 1
            editor.DrawText(pageNum & " of " & document.Pages.Count)
        Next 
    End Sub

How to reproduce:
public partial class Form1 : Form
{
    private DataTable dt = new DataTable();

    public Form1()
    {
        InitializeComponent();

        this.FillData();

        this.radGridView1.DataSource = this.dt;
        this.radGridView1.EnableSorting = true;
        this.radGridView1.EnableFiltering = true;

        this.radGridView1.MasterTemplate.DataView.BypassSort = true;
        this.radGridView1.SortChanged += radGridView1_SortChanged;
        this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
    }

    public void FillData()
    {
        dt.Columns.Add("Id", typeof(int));
        dt.Columns.Add("Name", typeof(string));

        for (int i = 0; i < 30; i++)
        {
            dt.Rows.Add(i, "Item" + i);
        }
    }

    private void radGridView1_SortChanged(object sender, Telerik.WinControls.UI.GridViewCollectionChangedEventArgs e)
    {
        if (e.Action == NotifyCollectionChangedAction.Add || e.Action == NotifyCollectionChangedAction.ItemChanged)
        {
            SortDescriptor s = e.NewItems[0] as SortDescriptor;
            string sortOperator = "";
            if (s.Direction == ListSortDirection.Ascending)
            {
                sortOperator = "ASC";
            }
            else
            {
                sortOperator = "DESC";
            }
            dt.DefaultView.Sort = s.PropertyName + " " + sortOperator;
        }
        if (e.Action == NotifyCollectionChangedAction.Remove)
        {
            dt.DefaultView.Sort = "";
        }
    }
}

Workaround: 
public partial class Form1 : Form
{
    private DataTable dt = new DataTable();

    public Form1()
    {
        InitializeComponent();

        this.FillData();

        this.radGridView1.DataSource = this.dt;
        this.radGridView1.EnableSorting = true;
        this.radGridView1.EnableFiltering = true;

        this.radGridView1.MasterTemplate.DataView.BypassSort = true;
        this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;

        this.radGridView1.CellBeginEdit += RadGridView1_CellBeginEdit;
        this.radGridView1.CellEndEdit += RadGridView1_CellEndEdit;
    }

    private void RadGridView1_CellEndEdit(object sender, GridViewCellEventArgs e)
    {
        if (e.Row is GridViewFilteringRowInfo)
        {
            this.radGridView1.MasterTemplate.DataView.BypassSort = true;
        }
    }

    private void RadGridView1_CellBeginEdit(object sender, Telerik.WinControls.UI.GridViewCellCancelEventArgs e)
    {
        if (e.Row is GridViewFilteringRowInfo)
        {
            this.radGridView1.MasterTemplate.DataView.BypassSort = false;
        }
    }
    public void FillData()
    {
        dt.Columns.Add("Id", typeof(int));
        dt.Columns.Add("Name", typeof(string));

        for (int i = 0; i < 30; i++)
        {
            dt.Rows.Add(i, "Item" + i);
        }
    }
}

To reproduce: please refer to the attached gif file and sample project.

Workaround:

        private void radGridView1_CellEndEdit(object sender, GridViewCellEventArgs e)
        {
            RadDropDownListEditor editor = e.ActiveEditor as RadDropDownListEditor;
            if (editor!=null)
            {
                RadDropDownListEditorElement el = editor.EditorElement as RadDropDownListEditorElement;
                el.AutoCompleteSuggest.DropDownList.ClosePopup();
            } 
        }

How to reproduce: check the attached video
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        
        this.radGridView1.DataSource = this.GetData(1000);
        this.radGridView1.AutoExpandGroups = true;
        this.radGridView1.EnableFiltering = true;
        this.radGridView1.EnablePaging = true;
        this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
    }

    private DataTable GetData(int count)
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("Id", typeof(int));
        dt.Columns.Add("Name", typeof(string));
        dt.Columns.Add("Date", typeof(DateTime));
        dt.Columns.Add("Bool", typeof(bool));

        for (int i = 0; i < count; i++)
        {

            dt.Rows.Add(i,"Name " + i,  DateTime.Now.AddDays(i), i % 2 == 0 ? true : false);
        }

        return dt;
    }
}

Workaround: cancel the PageChanging event
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        
        this.radGridView1.DataSource = this.GetData(1000);
        this.radGridView1.AutoExpandGroups = true;
        this.radGridView1.EnableFiltering = true;
        this.radGridView1.EnablePaging = true;
        this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

        timer = new Timer();
        timer.Interval = 100;
        timer.Tick += (sender, e) =>
        {
            timer.Stop();
            this.shouldCancel = false;
        };

        this.radGridView1.PageChanging += RadGridView1_PageChanging;
        this.radGridView1.CurrentRowChanged += RadGridView1_CurrentRowChanged;
    }

    private void RadGridView1_CurrentRowChanged(object sender, CurrentRowChangedEventArgs e)
    {
        this.shouldCancel = this.ShouldCancelPageChange(e.CurrentRow);
        timer.Start();
    }

    Timer timer;
    bool shouldCancel = false;

    private bool ShouldCancelPageChange(GridViewRowInfo rowInfo)
    {
        if (this.radGridView1.TableElement.MasterTemplate != null && this.radGridView1.TableElement.MasterTemplate.EnablePaging)
        {
            int pageIndex = this.radGridView1.TableElement.ViewTemplate.DataView.GetItemPage(rowInfo);

            if (pageIndex == this.radGridView1.TableElement.MasterTemplate.PageIndex)
            {
                return true;
            }
        }

        return false;
    }

    private void RadGridView1_PageChanging(object sender, Telerik.WinControls.PageChangingEventArgs e)
    {
        e.Cancel = this.shouldCancel;
    }

    private DataTable GetData(int count)
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("Id", typeof(int));
        dt.Columns.Add("Name", typeof(string));
        dt.Columns.Add("Date", typeof(DateTime));
        dt.Columns.Add("Bool", typeof(bool));

        for (int i = 0; i < count; i++)
        {

            dt.Rows.Add(i,"Name " + i,  DateTime.Now.AddDays(i), i % 2 == 0 ? true : false);
        }

        return dt;
    }
}

How to reproduce: check also the attached video
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        this.radGridView1.DataSource = this.GetData();
        this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

        GroupDescriptor descriptor = new GroupDescriptor();
        descriptor.GroupNames.Add("Name", ListSortDirection.Ascending);
        this.radGridView1.GroupDescriptors.Add(descriptor);
    }

    private DataTable GetData()
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("Name", typeof(string));
        dt.Columns.Add("Age", typeof(int));
        dt.Columns.Add("Date", typeof(DateTime));
        dt.Columns.Add("Bool", typeof(bool));
        for (int i = 0; i < 3; i++)
        {
            for (int j = 0; j < 60; j++)
            {
                dt.Rows.Add("Name " + i, i, DateTime.Now.AddDays(i), i % 2 == 0);
            }

        }

        return dt;
    }

    private void radButton1_Click(object sender, EventArgs e)
    {
        GridPrintStyle style = new GridPrintStyle();
        style.FitWidthMode = PrintFitWidthMode.FitPageWidth;
        this.radGridView1.PrintStyle = style;

        RadPrintDocument doc = new RadPrintDocument();
        doc.HeaderHeight = 30;
        doc.HeaderFont = new Font("Arial", 12);
        doc.LeftHeader = "Left Header";
        doc.MiddleHeader = "Middle header";
        doc.RightHeader = "Right header";
        doc.AssociatedObject = this.radGridView1;

        RadPrintPreviewDialog dialog = new RadPrintPreviewDialog(doc);
        dialog.Show();
    }
}

Workaround: create a custom GridPrintStyle class
public class MyGridPrintStyle : GridPrintStyle
{
    protected override BaseGridPrintRenderer InitializePrintRenderer(RadGridView grid)
    {
        BaseGridPrintRenderer rend = base.InitializePrintRenderer(grid);
        if (rend is TableViewDefinitionPrintRenderer)
        {
            return new MyTableViewDefinitionPrintRenderer(grid);
        }

        return rend;
    }
 
}

public class MyTableViewDefinitionPrintRenderer : TableViewDefinitionPrintRenderer
{
   
    public MyTableViewDefinitionPrintRenderer(RadGridView grid)
        : base(grid)
    {
    }     

    protected override void PrintRowWideCell(GridViewRowInfo row, TableViewRowLayout rowLayout, GridPrintSettings settings, int currentX, int currentY, Graphics graphics)
    {
        int groupLevel = row.Group != null ? row.Group.Level + 1 : 0;
        int indentLevel = row.HierarchyLevel + 1 - groupLevel;
        Size cellSize = this.GetRowSize(row, rowLayout);
        int cellX = currentX + indentLevel * settings.HierarchyIndent + rowLayout.Owner.CellSpacing;
        Rectangle cellRect = new Rectangle(cellX, currentY, cellSize.Width - indentLevel * settings.HierarchyIndent, cellSize.Height);
        CellPrintElement printCell = new CellPrintElement();

        if (row is GridViewGroupRowInfo)
        {
            if (this.PrintPages.Count > 0 && !settings.PrintHierarchy)
            {
                cellRect.Width -= this.PrintPages[this.CurrentPrintPage].Count - 1;
            }

            printCell = this.CreateGroupCellPrintElement(row as GridViewGroupRowInfo);

            if (printCell.Font != settings.GroupRowFont)
            {
                if (settings.GroupRowFont != null)
                {
                    printCell.Font = settings.GroupRowFont;
                }
                else
                {
                    settings.GroupRowFont = printCell.Font;
                }
            }
        }

        printCell.TextPadding = this.GridView.PrintStyle.CellPadding;
        printCell.RightToLeft = this.GridView.RightToLeft == RightToLeft.Yes;

        PrintCellFormattingEventArgs formattEventArgs = new PrintCellFormattingEventArgs(row, null, printCell);
        this.OnPrintCellFormatting(formattEventArgs);


        PrintCellPaintEventArgs paintEventArgs = new PrintCellPaintEventArgs(graphics, row, null, cellRect);
        this.OnPrintCellPaint(paintEventArgs);

        formattEventArgs.PrintCell.Paint(graphics, paintEventArgs.CellRect);

    }
}

Currently the time for opening the popup for a column with 100 000 unique items including blank items is about 8s, this can be optimized.

How to reproduce: 
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        this.radGridView1.EnableFiltering = true;
        this.radGridView1.ShowHeaderCellButtons = true;

        this.radGridView1.DataSource = this.GetData();
        this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

        this.radGridView1.MouseDown += RadGridView1_MouseDown;
        this.radGridView1.FilterPopupInitialized += RadGridView1_FilterPopupInitialized;
    }

    Stopwatch sw = new Stopwatch();

    private void RadGridView1_MouseDown(object sender, MouseEventArgs e)
    {
        GridFilterButtonElement btn = this.radGridView1.ElementTree.GetElementAtPoint(e.Location) as GridFilterButtonElement;
        if (btn != null)
        {
            sw.Start();
        }
    }

    private void RadGridView1_FilterPopupInitialized(object sender, FilterPopupInitializedEventArgs e)
    {
        ((RadListFilterPopup)e.FilterPopup).PopupOpened += Form1_PopupOpened;
        
    }

    private void Form1_PopupOpened(object sender, EventArgs args)
    {
        sw.Stop();
        Console.WriteLine("Total: " + sw.ElapsedMilliseconds);
    }

    private DataTable GetData()
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("Id", typeof(int));
        dt.Columns.Add("Name", typeof(string));
        dt.Columns.Add("Date", typeof(DateTime));
        dt.Columns.Add("Bool", typeof(bool));
        for (int i = 0; i < 100000; i++)
        {
            dt.Rows.Add(i, "Name " + i, DateTime.Now.AddDays(i), i % 2 == 0);
        }


        dt.Rows.Add(1, null, DateTime.Now.AddDays(1), false);
        return dt;
    }
}