Now, I'm not sure if the problem is really 100% fixed, but at least now the lags are much smaller- and here's how to do it (kudos to matthewls and vrenn):
- Add to /etc/xorg.conf in the Device section
Option "TripleBuffer" "True"
- Create a file in /etc/profile.d with content
__GL_YIELD="USLEEP"(yes that starts with a double underscore).
- Log out, stop your display manager, restart it.