Unfortunately there's not much we can do about this particular scenario. The whole performance issue comes from 1) rendering and animating a DIV on top of the Silverlight plugin and 2) marking the Silverlight plugin as Windowless. Let's hope that future versions of the Silverlight plugin will provide better HTML integration.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items