I made a site at 20fps on the mac with flash 5. It runs fine on mac, but when testing it on a pc with flash 5, it runs about 40% faster. Nothing to do with processor speed, it just plays too fast.
As a test, I also made a 6 second movie at 20fps. On a mac, it took 8.3 seconds to finish. On a pc it took 6 seconds. the movie was very simple so it didn't matter if you were on a fast computer or not..