Don't know but something's not accelerated enough obviously. So how about Option "RenderAccel" "true" in xorg.conf, and perhaps some more stuff, don't know:
Code:
Section "Screen"
<snip>
Option "RenderAccel" "true"
Option "AddARGBGLXVisuals" "true"
Option "AllowGLXWithComposite" "true"
Option "TrippleBuffer" "true"
SubSection "Display"
Depth 24
EndSubSection
EndSection
Section "Extensions"
Option "Composite" "Enable"
EndSection
Hope some help.