The first thing that comes to my mind is that 50 columns with 100 rows are 5000 cells and if you have a lot of visible cells (cells are small or the pane is big) or you placed the RadGridView into a StackPanel or other panel that breaks its virtualization, it is normal to take a long time for WPF to lay out all these cells.
Another possible reason for the delay is if you are binding the ItemsSource of the RadGridView to the DataContext of the pane.
If you could give more details about the case you have and eventually a sample project reproducing the problem, this would be of great help for us.
All the best,
the Telerik team