I had also the same problems with last Xorg 1.11 and nvidia GeForce G105M card. This card seems do not supports trapezoid acceleration checked with "nvidia-settings -q AccelerateTrapezoids".
For impatient, like me, I made and attached a dirty and quick fix for Xorg 1.11 from AaronP explanation in http://lists.x.org/archives/xorg-dev...er/026050.html
which seem to fix the problem in wait a real and serious fix from xorg or nvidia guys.
WARNING ! this patch is not an official patch and do not pretend to fix the slowdown bug, it's only a quick and dirty hack