This question already has an answer here:
I have serious performance issues with flash on ubuntu 11.10. the problem manifests itself when I try to play large flash files on hosting sites like gorillavids.com and the like. the playback starts normal, and then after 10 minutes or so the frame rate keeps getting lower and lower, until it's like 2 fps. the whole file gets loaded and audio is fine. this happens on my main rig (i5, 8 gb, radeon hd5450) and on my laptop (i3, 4 gb, intel hd graphics). after booting to windows, the files play fine. I haven't had any issues with youtube vids, since they're generally not that long.
I've tried it with chromium and firefox with the flash player from restricted and with adobe's flash player, everything behaves the same.