I noticed that YouTube videos playing full-screen in HD (720p or 1080p) are all jittery with missing frames and slightly desynced audio/video, in other words, playback is not smooth (HTML5). I tried vasrious older versions like 22.214.171.1240 and 126.96.36.1990 (portable editions) and those have the same problem with YouTube. I tried Firefox and of course, that one worked perfectly.
I then turned on Retro Mode and that seems to have helped.
So, is Ultra Mode broken for full-screen YouTube HD playback in 4.9 Maxthon branch ?