The reason for the performance issues is the size of the ItemsSource for the combo column and the internal lookup logic.

A new IsLightweightModeEnabled property of GridViewComboBoxColumn will be introduced with R2 2016. Its default value is False. 
When set to True, the new lightened lookup logic is triggered, thus the performance of the column is significantly increased.

I want to get ItemsSource by retrieving a list from a dictionary, so I need 2 parameters, but I can't use MultiBinding to do this!

Update: Before our change the GridViewGroupFooterCell content was right aligned and the GridViewFooterCell content was left aligned. We decided to unify them by setting default left alignment to both. This can be easily modified by changing the Column FooterTextAlignment property to Right or by using the following implicit styles: 

<Style TargetType="telerik:GridViewGroupFooterCell" BasedOn="{StaticResource GridViewGroupFooterCellStyle}">
    <Setter Property="HorizontalContentAlignment" Value="Right"/>
</Style>
<Style TargetType="telerik:GridViewFooterCell" BasedOn="{StaticResource GridViewFooterCellStyle}">
    <Setter Property="TextAlignment" Value="Right"/>
</Style>

The fix is available in R3 2017 of UI for WPF.

There is a mistake in the text shown in the GroupPanel of RadGridview in french.
it should be something like "Glissez un en-tête de colonne et déposez-le ici.."

The problem should be resolved with lib version 2015.3.1019.

It should be resolved with lib version 2015.1.504.

When Unicode characters are copy pasted from grids via CTRL + C and then pasted into Excel or Word (for example) as a HTML table, it seems like for every Unicode character a certain amount of bytes / characters are cut off from the end result.

StackOverFlowException when the GroupMemberPath is set to a different path to the data member and the column is grouped, and filtered.

As an example I add a column to the grid:
columns.Add(new GridViewDataColumn() { DataMemberBinding = new Binding("CustomerId"), GroupMemberPath = "CustomerName", SortMemberPath="CustomerName", Width = 300 });

When the grid appears, drag the CustomerId column to the grouping area
Click the filter icon on the CustomerId column
Click a checkbox in the filtering control to select an item
Exception occurs

The problem should be resolved with lib version 2016.1.125.

When dealing with Date column filters show available options (similar to text filters) and support grouping based on Year, Month. This is a very handy feature of Excel.