Jaime Rodriguez On Windows Phone, Windows Presentation Foundation, Silverlight and Windows 7
Seema shares some of her lessons learned towards minimizing CPU usage in silverlight .. http://blogs.msdn.com/seema/archive/2007/08/09/silverlight-a-few-thoughts-on-minimizing-cpu-usage.aspx
I can vouch first hand for her first two as big winners... There are some extra tips in the RC docs: http://msdn2.microsoft.com/en-us/library/bb693295.aspx
One last tip: test perf on both Mac and windows (Xp plus Vista). there are slight variations..
If you have better tips.. please leave comments :)