The old man in me wants to ask: if that's supposed to be a concrete advantage for normal users, why can't those normal users concentrate on that video, but have it both playing (wasting resources) and scroll around ignoring it?
It's just embarrassing. Picture this, your windows-using friend is considering trying Linux, and you go on demonstrating your distro of choice to them... you open the browser, go on a youtube page, and what do they see? A video that doesn't stay in sync with the browser windows - now that's just unprofessional. They'll be all like "what's up with this glitchy video" and you'll be forced to explain that the display server is so bad that it's impossible to fix it...
Except that itís not happening here, so looks like itís possible to fix itÖ
No, the basic architecture of X11 makes it impossible to fix without fundamental changes. Your system may be fast enough that you don't notice it, but the underlying problem is still there. And "get a faster computer" is not a good response to fundamental design flaws, not when there is an alternative that lacks those flaws.