Strange. I have an Athlon Thunderbird running at 1.1 Ghz, 256 mb ram and the game is normal speed. The only thing that's off a bit is the Double-Swing in first person mode (the spark hits and animations are out of synch with each other).
I'm running in 1024x768, no back buffer in system memory, GeForce2 GTS (32mb) card, screen size 9....
There's a console command line to further tweak display settings but you'd have to go to Massassi.net to look it up, I don't remember what it was.
Perhaps you could try reinstalling and applying the patch, but I don't know if that would do anything.