According to this mesa ML discussion it looks like torcs has some performance bottlenecks:
http://lists.freedesktop.org/archives/mesa-dev/2011-December/016219.html
http://lists.freedesktop.org/archives/mesa-dev/2011-December/016524.html
http://cgit.freedesktop.org/mesa/mesa/commit/?id=2a904fd6a0cb80eec6dec2bae07fd8778b04caf3
That was improved in recent mesa but maybe torcs can anyway be improved to run better also on older mesa or on other drivers.
Interesting, thank you for th info.