1) Close all background tabs that have activity going on.
2) Stop the video immediately and only hit Play again when the page has fully loaded and the video has fully buffered.
3) Try reducing the video resolution to 144p and then go up to the maximum resolution you find acceptable performance-wise.
4) Add this to the video URL:
This will give you the classic YouTube layout (as long as it's still available), which doesn't help with video performance per se, but the page loads a lot quicker.
Safari 5 with the latest version of Leopard Webkit (10.5 only) has no problems playing YouTube videos. It also struggles with the new layout, but since it has hardware video support it will simply start playing the video smoothly even while the processor is trying to cope with the page layout.