Well, I set up my system with Ubuntu minimal (console only, 9.10) to get a nice boot time. I then installed an XServer and Fluxbox:
After that completed I started fluxbox ("startx") and tried glxgears which gave me over 8000 frames at 5s.
sudo apt-get install xorg fluxbox mesa-utils
But then rebooted the system and since then I'm stuck at 900 frames at glxgears (and also very slow performance at other OpenGL applications). I also tried the normal Ubuntu installation, but there everything is fine.
Btw. I use an Radeon Moblity 9600 with the open source driver "radeon". I already compared the xorg log and the loaded kernel modules before and after the first restart. Both are identical.
So what went wrong and how to resolve it? Any ideas?