When a RadTreeView with lot of items (800+) is placed in an Accordion from the Microsoft Silverlight Toolkit, there is a performance issue. More specifically when we try to collapse the accordion, the application freezes for few seconds. Also note that the IsVirtualization property is set to True. The XAML team has recently reviewed this issue and will not be addressing it as at this time the team is focusing on the bugs impacting the highest number of developers. If you have encountered this issue and it is blocking for your work please contact us through the support ticketing system with details on your setup and a reference to this item.