If you load many items in the ItemsSource, than opening the ComboBox takes long time. I have rechereched the internet and found out that the VirtualizingStackPanel does not support grouping. This feature has been introduced in .NET 4.5 [VirtualizingPanel.IsVirtualizingWhenGrouping](http://msdn.microsoft.com/en-us/library/system.windows.controls.virtualizingpanel.isvirtualizingwhengrouping.aspx).
Is there a way to use the VirtualizingStackPanel only when compiling the 4.5 project and to use the ItemsPresenter otherwise.