I have multiple monitors (four) and I like to have a YouTube video running in one of them, and what I found was with some videos (not nearly all of them) was that the computer would become so slow it was essentially unusable, and I couldn’t figure out why.
I thought at first that it was the quality of the video, so I would try the lowest quality, but it was no different.
I tried uninstalling things, and changing my flash plugin in Chrome, to no avail.
The solution was googling “HTML 5 VS flash chrome youtube”. What I came to was youtube’s webpage at http://www.youtube.com/html5.
It showed at the bottom that I was currently in the HTML 5 trial. Maybe I agreed at some point, maybe not, but I thought, “hey, lets turn it off”.
Sure enough, it worked. No more slow down. So it appears that YouTube still has some bugs in their HTML 5 video player!
Hope this helps you!