Why does Safari freeze on YouTube HTML5 videos?
I had a similar problem with my Safari. I went into /Library/Internet Plug-Ins and removed the two piece of Flash cruft (regretfully I'm at work, so I don't the exact file names.)  After a reboot, Safari worked much better, as did Chrome.  Not sure why Chrome improved, but it improved most of all.

Incidentally, I thought I was on a Flash free system previously, so I'm not sure how those bits got there.  If I need Flash, I just go to Develop -> Open Page With -> Chrome (you need to enable the Developer Menu, under Advanced in Preferences.)

Hope this help