I've just installed Ubuntu 12.04 yesterday, and i'm having problems watching video's on YouTube. Once I get to full screen the frame rate drops to 16fps. My PC runs the latest games on very high, so I don't think my hardware would be a problem.
I'm using Chromium, but Firefox has the same issue.
Other flash based streaming videos work fine (like twitch.tv). It's the same low frame rate between HTML5 and Flash.
Basically it's a Youtube problem and not a flash issue.
Sadly Adobe Flash 11.2 has a blacklist of graphics cards which it will not allow to use hardware acceleration, even with a good card, so it's just luck if yours is supported and on the white list...
You could try installing MiniTube and gstreamer0.10-ffmpeg which allows you to search and play YouTube videos full screen using system codecs instead of the flash player...
I think I found it. It's because google chrome using 2 flash plugins.
Open up Google Chrome and in the address bar type chrome://plugins/
You will then see a Details link over to the right-hand side of the page. Click the + and you’ll see a long list of the installed plugins in detail.
Look in the Flash section and find the line which has the text ../pepflashplayer.so Click the Disable link next to this line and restart the browser.
In last step I actually need only reload YouTube page :)
Full description: http://rextronics.co.uk/general/how-to-fix-jerky-full-screen-youtube-videos-in-google-chrome/
Try using SMPlayer with the SMTube, which will allow you to watch YouTube videos without using HTML5 or Flash (it uses MPlayer, which is much faster than Flash). You can try a number of output drivers, to see which one works best (xv works great for me).
To install SMPlayer, run the following
sudo add-apt-repository ppa:rvm/ppa
sudo apt-get update
sudo apt-get install smplayer smtube
Solved the problem in Firefox by switching layers.acceleration.disabled to false and the force option to true.
