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 focuses only on pure computational speed (and measures the time taken to compute 10000000 prime numbers).

And here are the results of the test run by the authors of the benchmarks (green the fastest, red the slowest):

UPDATE: Removed the table with the results as it will be an infringement of Silverlight license (not allowed to publish benchmarks of beta software)

From a quick look at the figures it seems that:

• Flex is way faster than Silverlight when rendering complex UI.
• Flex and Silverlight have pretty much the same speed with just animation.
• Silverlight is almost 4 time faster than Flex in pure computation speed.

I was not able to run all the tests myself, and the version on Silverlight 2 is implemented with Beta 1 and doesn’t run with the beta2 plugin.

Anyone has the time/will to try and optimize the code used in the GuiMark?