Using Windows 98
The PC spec isn't brilliant - 400MHz Celeron (actually 2 of these, but only NT + Linux use both), 256Mb RAM, Matrox G400Max.
Without sound the framerate is very healthy, although the graphics are not at maximum detail. Typically 30fps or more, and I've never noticed any slowdown. This is also true for the training missions with sound. However, when the sound is on and people are shooting at me I'm getting 0.5 - 0.9fps.
Seems like there must be some glitch in the shooting effects.
I'll grab the latest drivers and see what that does, although I doubt they've been updated in quite a while - I got the card more than four years ago.