http://bubblemark.com/ has a series of performance tests for RIA applications.  Here's the results of my test run tonight on my Dell D820 Centrino Duo 2GHz with integrated graphics. 

Silverlight 1.1     = 104 fps (CLR)
Silverlight 1.0     = 58 fps (JavaScript)
Flex (cached)       = 64 fps
XBAP                = 63 fps
DHTML               = 63 fps
Flex                = 55 fps

The performance of the CLR-based Silverlight app was significantly better than all of the others, including the Silverlight JavaScript version.  W00t!