I have an ATI Radeon 9800 XT, with an R350 chipset. I use the open source drivers (the ones that comes by default, "radeon" or "ati" in the xorg.conf file), and I used to have horrible performance. [...]

But, I found a solution.

In the Device part of your xorg.conf, add the following line:

Code:

Option "XaaNoOffscreenPixmaps" "true"