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.