Indeed with the current version of RadGridView there is a significant performance hit with grouping more levels (>3) and when AutoExpandGroups is true. The problem comes from the fact that in that case RadGridView iterates all items (groups and subgroups) in order to calculate a proper extend height (used for scrolling). We have plans to sacrifice calculating this information and this will result in a great performance boost while scrolling with such cases will be a little bit weird (but expected). What I mean initially only visible groups will be expanded (so only their items will be iterated), then when scroll reached new group this group will automatically expand due to AutoExpandGroups property value but this will reflect in a greater extend height and scroll thumb will jump a little.
Same behavior like doing this manually - expand first group and scroll to the end, second group is not expanded while expanding it will enlarge extend height. We are doing our best to provide this optimization as soon as possible, but I cannot commit with any specific date or release.
Sorry for the inconvenience caused.
the Telerik team
Browse the videos here>>
to help you get started with RadControls for Silverlight