Currently the RadTileView doesn't support virtualization, so it generates all its items in the beginning and if you have a lot of tiles or they have a lot of items inside them that process will take a while.
At the moment, however, we are working on a UI Virtualization support (it'll be ready for the Q1 2011 Beta, sometime in January) for the RadTileView which will improve the performance and hopefully solve your issue.
If you have further questions feel free to ask.
the Telerik team
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to help you get started with RadControls for Silverlight