The answer is: it depends. Flash/Flex is faster when it comes to graphic rendering, but Silverlight is faster when it comes to pure computation speed.
There are two speed benchmarks for RIA application, and they both focus on UI rendering:
BubbleMark: it displays many bubble floating around the screen as fastest as possible, and use the resulting frame per second as result of the test
GUIMark: similar to the BubbleMark, but with more effects and animations used: vectorial images, alpha transparencies, text reflow and so on.
Then another benchmark has been developed, CountingPrimes, which...