An open mplayer outputing to an xv surface slows down other 2D operations, even when mplayer is on pause (and top shows no cpu consumption by X or mplayer).

Scrolling in my gmail account, or some more complex pages like:
http://www.one.de/shop/one-notebooks-c-213.html

show can't-keep-up-with-painting effects when scrolling with FireFox-3.5, although scrolling worked fine before. Although scrolling is slow, neither X nor FireFox show high CPU consumption. Even after closing mplayer performance is not back to normal.

I see this problem on an i945GM, kernel-2.6.31.rc1 and intel-2.7.99 and xorg-1.6.99.
Sysprof shows 45.6% of cycles in firefox (5% in kernel), and 40% of cycles in xorg (18% in kernel). Xorg spends 11% in RenderTrapezoids and 4% in RenderComposite.