A great post about some Silverlight performance tips: Silverlight: A few thoughts on minimizing CPU usage [1]:
The first two suggestions will have the most drastic improvement on the performance of your Silverlight application, and can affect CPU usage, framerate, and application responsiveness.
<li><em>IsWindowless=false is faster</em></li> <li><em>Opaque Background is faster</em></li> <li><a title="Read full story..." href="http://blogs.msdn.com/seema/archive/2007/08/09/silverlight-a-few-thoughts-on-minimizing-cpu-usage.aspx">Read full article...</a></li>
An there is an MSDN article about Animation Performance Tips [2].