Same problem on my macbook air! I have tried firefox, safari, chrome and on all of them I get choppy video after a while and cpu jumps up to 70-80 percent. I have contacted applecare without any success!! I hope that someone take the responsibility and fix the mac problem with flash.
I have the same problem. I have an older (first gen) Macbook Pro and I'm experiencing very slow playback with 720p flash video. Browser CPU usage jumps to 120-140% as soon as I start playing a video (youtube, vimeo, etc.) This doesn't seem like a performance issue due. I can play multiple gigabyte HD videos fine with Quicktime or VLC, but flash video is very slow.
This was with 10.6. I downgraded to 10.5.8 after a clean install and after performing update the playback is still very laggy, slow and stuttering.