YouTube recently switched to a widescreen format and while browsing around to test out the quality I was still shocked at the poor performance of Flash on a Mac, specifically with video. The playback is jerky and constantly pauses in both Firefox and Safari. Restarting the browser helps a bit but by no means solves the problem.
I had recently downloaded Flash Player 10 (10,0,12,36 at the time of this post) and noticed a slight improvement in playback and performance but still not what I would consider acceptable for video. Ars Technica published an article in October benchmarking Flash Player 10. On a Mac Pro, Flash 10 used between 25% and 75% of CPU resources. On the same hardware running Windows it used 6% CPU. On my MacBook Pro 2.33GHz I see between 30%-45% CPU usage watching YouTube. The difference in performance here is enormous and I assume is the result of a product coded for Windows first and then ported to the Mac.
I haven't been able to find any peformance tweaks that would improve video playback but I did find a bug reported against Flash 10 for poor performance on the Mac in the Adobe Flash bug tracker. Register a bug tracking account and vote it up please.