Guys, you're not going to believe this...
I have to explain. I wrote a distributed program which can brute-force MD5 hashes (to be more precise, such of BSD-style password encryption) on pretty much any number of computers. I fed the server with test data and installed clients on all of my machines (to be run on boot-up), of course nice'd such that it does not interfere with other applications running on the machines.
Well, this was no problem at all as long as, right, I never tried glxgears.
Now, for some bloody reason, glxgears got nearly no cpu time while that client was running (altough glxgears had priority 0 and that client priority 5), thus had framerates around 15-30 fps...
I discovered this by accident, while I was trying quake3, because I wasn't quite sure if glxgears was the right application to test 3d functionality. Q3 was also severely slowed down by that client running in the background, but I knew from experience that software rendering had to be much slower than q3's frame rates, so I killed the client et voilą, everything worked fine
Shame on me and sorry for the trouble (well, at least you all had a laugh)
nice -n 19 and everything works fine