Comment # 7 on bug 1134805 from Stefan Dirsch

Wow! Still a 32bit system with Radeon RS400 gfx card. This card is still from
2005! 

You may want to have a try with xf86-video-ati package uninstalled to force X
run with modesetting driver. But I'm afraid this may make things even worse
with glamor (2D acceleration via Mesa 3D driver). So next thing to try then
would be:

Section "Device"
  Identifier "Default Device"
  Driver "modesetting"
  Option "AccelMethod" "none"
 EndSection

in /etc/X11/xorg.conf.d/50-device.conf. Le me know whether this helps ...