Even though the browser support HTML 5 video streaming the actual decoding of the stream is done using CPU as there is no implementation for the browser to use the build in hardware decoders.
I think the other problem is that desktop is also software rendered? (referenced from this forum post) So its an amplified problem where everything starts to chop out of control.
If you are streaming a high quality YouTube video then you will get this choppy'ness due to CPU struggling to keep up with rendering it to screen.
Kodi and XBMC have support for direct hardware decoding (and OpenGL UI rending) that is why videos can stream from Plex and other places at acceptable rates. (POssibly even plugins like YouTube)