Comment 2 for bug 315267

Actually, this problem not seems to depend on exact card model or something like this (people with at least ATI and Nvidia encounter this in various Linux distros including Ubuntu, Kubuntu and whatever else).

And actually there is nothing wrong with X server. It is obvious that usual output method was never intended to draw hi-resolution video so it's slow and this is "by design". About accelerated video... as far as I know, there is something like this happens: when composite mode used, screen updates are no longer synchronized with display frame rate. This causes no troubles to usual windowing operations but video displayed through accelerated interfaces (xvideo, opengl, ...) blinks and tears. Blinking and tearing happens since when composite mode used it happens that video frames are output without syncing to monitor frame rate. Hence it happens that frames could be just partially drawn when getting actually displayed. This surely leads to nasty blinking due to repeated sequence of partially-drawn frames.

Here goes verbose lspci log (launched under sudo since without it it lacks some info).