I noticed this issue on a few computers:
My own PC: quad core Intel 6600, Nvidia GT250, 8gb ram
Also i used to test this in a few work computers, but i unfortunatelly cant say concrete specs.
and on Lenovo Miix 2 Tablet, with quad core Intel BayTrail z3740 and 2gb ram.
Same performance everywhere.
Stutter/judder/microlag/choppy when you use default and widescreen modes.
No matter what fps HD video i watch, 30 fps gets a bit better performance but still have notable stutter, videos with 25 fps and below looks even worse. But when i turn on full screen mode any videos with any fps goes smooth. HTML5 does not have this problem.
I assume that maybe you get additional loads on flash player when it plays from youtube site, there is many elements that can in theory interrupt video performance.
Can someone help us find the right answer?