Experiencing the same issue on both my desktop computer running on gtx760 and on my laptop with GT 520MX. I noticed its not only on fullscreen video, but also when playing youtube video on the whole maxthon window (to do this, I right clicked a video for embeded link and got link iin format "www.youtube.com/embed/xxxxyyzzz"). This way it is running slower, but not nearly as when playing video in fullscreen. Really annoying problem, I have to open another browser to view vids fullscreen. The only way around this is apparently to install youtube center and force flash video playback.