Perhaps this improvement belongs in TGroup or some other class than TMesh, i'm not sure.
My collision detection code calls TAvatar >> extent fairly often (at least once per avatar per collision candidate per frame). With only a handful of collision objects and a very fast computer, this knocks the framerate down from 48Hz to 12Hz! I put in a simple modification to TAvatar to just store its extent and only recalculate it every N requests (N=1000), and with this modification the frame rate is rock steady at 48Hz.
However, recalculating every 1000 requests is a shoddy solution, it should recalculate *if it's changed* since the last request, which i'll look at implementing, altho i think someone more familiar with the architecture would probably do a better job.
Thanks to Bert's Blog for pointing out the MessageTally profiler, which singled out this problem immediately.