For the last few months, every youtube video I have tried to watch in Safari was slow, jerky and almost hangs Safari.
I would always have to switch to a different browser to watch youtube (all other flash videos worked fine).

Then I found this:
https://discussions.apple.com/thread/3834680?start=0&tstart=0

By simply leaving the HTML5 experiment, youtube videos in Safari started working 100% perfectly again.

My setup is:
Macbook Air, Mid 2011
1.7GHz, Core i5
4GB RAM
Intel HD 3000 graphics
OS 10.7.3
Flash player: 11.3.300.257