It's hard to describe my surprise that my 5-year-old PC, with a low power 1.6Ghz CPU, can easily breach 100fps while playing JamLegend, and my 9-month old, dual-core 2.66Ghz MacBook Pro can barely squeak out 40fps.
Speaking of framerate in Flash, setting the framerate on a particular SWF does not actually make the SWF run at that framerate, even if the client machine can run at that framerate. Rather, the framerate seems to be more a measure of CPU resources dedicated to running the SWF.
For example, back to JamLegend: My MBP can run JamLegend at 40fps, but not if the framerate is set to 40fps. If it's set to 40fps, I'll get 20. It it's set to 60fps, I'll get 30. In order to get 40fps, the framerate has to be set to 100. Meanwhile, on my old XP machine, setting it to 40 gets 40, 60->60, and 100->100.
So, very good question, why is it that Flash performance is so different on OS X than Windows?