To reproduce:
Add a RadGridView and use the following code. When you expand the first row, the vertical scrollbar is not correct. Scrolling down changes its size. However, if you expand the last row, it takes few seconds to open.

Second scenario:
In addition of the following code, if you change the TableElement.PageViewMode to PageViewMode.ExplorerBar, expanding the first row takes few seconds to load the hierarchical data as well.

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

        List<Item> items = new List<Item>();
        List<SubItem> subItems = new List<SubItem>();

        for (int i = 1; i <= 3; i++)
        {
            Item item = new Item(i, "Item" + i);
            
            for (int j = 1000; j <= 2010; j++)
            {
                subItems.Add(new SubItem(j, "SubItem" + j, i));
            }

            items.Add(item);
        }

        this.radGridView1.DataSource = items;
        this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

        GridViewTemplate template = new GridViewTemplate();
        template.ReadOnly = true;
        template.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

        GridViewRelation relation = new GridViewRelation(this.radGridView1.MasterTemplate, template);
        relation.ParentColumnNames.Add("Id");
        relation.ChildColumnNames.Add("ItemId");
        this.radGridView1.MasterTemplate.Templates.Add(template);
        this.radGridView1.Relations.Add(relation);
        template.DataSource = subItems;
    }
}

public class Item
{
    public int Id { get; set; }

    public string Title { get; set; }

    public Item(int id, string title)
    {
        this.Id = id;
        this.Title = title;
    }
}

public class SubItem
{
    public int Id { get; set; }

    public int ItemId { get; set; }

    public string Name { get; set; }
    
    public SubItem(int id, string name, int itemId)
    {
        this.Id = id;
        this.ItemId = itemId;
        this.Name = name;
    }
}