I can't play HD videos from Youtube or Vimeo smoothly with Flash Player. (Flash 10.0.45.2, Windows XP SP3, latest ATI drivers)
The framerate is low and sometimes the sound and the video hangs. The problem is the same on Firefox 3.6 and Internet Explorer 8.
When I download the videos (with DownloadHelper) and open them with Media Player Classic and the FFDshow codec, it works perfectly. Unfortunately FFDshow can't be used for streaming, I always have to download the video first.
It seems that FFDshow is much faster than Flash Player.
Here's a vido who hangs on Flash Player but not with FFDshow : http://vimeo.com/8614771