Hey, After formatting and reinstalling my MBA, I noticed that YouTube5 extension for Safari didn't work properly. Most 720p and 1080p video lags so much that I can't even watch them. Weird thing is that I can watch them flawlessly and smoothly with Flash (10.2) and that this only happens when I switch to fullscreen. If you let windowed, even running on 1080p video still plays fine. I don't remember having this issue before reinstalling so it is possible that some "core driver" is missing? I also made a PRAM and SMC reset... Help please, I'm really desperate, I just got used at HTML5 and don't wanna go back to Flash. Thanks! Edit. I did what you said and here are my tests results: Safari with YouTube HTML5 activated could watch flawlessly this video: http://www.youtube.com/watch?v=XITHbsUUlYI at 720p but not with 1080p. Safari with YouTube5 extension couldn't even watch both 720p and 1080. Safari with flash 10.2 could watch both smoothly 720p and 1080p Thing is that I don't understand why in the past I could watch all the videos I wanted at fullscreen regardless if it was either 720p or 1080p using YouTube5. Is it possible that my Mac got some kind of bottleneck or it's blocking its CPU usage?