Looking at your xaml I found a setting where you turn off the row virtualization .
Please try turning it back on and let me know if this makes any difference.
*As a side note : Sometimes is hard to read XAML form a jpeg image. In case you need to paste more XAML , you can use the "format code block " button in the editor when posting in the forum and paste the XAML as a text , rather than as a picture.
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
for a walk through all the exciting stuff we ship with the new release!