One thing that I have noticed has improved performance on two machines that my friends have, has been changing the sound option from it's default Miles Fast 2D, to Directsound Hardware (helped stopped the game lagging).
The only tweak I use with the game, is the NO CD crack, and all of the audio folder copied to my HD to eliminate any of the Music loading time.
I have set 2 different copies of this game up on 2 friends computers as well, the first one had a:
P3 500, GF2MX400 w/64 ram, SB PCI 128, 196ram, Win98se
The 2nd one had a:
P3-850, GF2 GTS, SB Live, Win98, 256 ram
On both of these systems the game started to run fine, once D3D Hardware sound was selected, and draw distance turned down to halfway at 800x600, the latter system could manage 32bit, the former only 16.
The first system slows down significantly when the game rains, as well as in large police chases. Resolution, graphics settings make little difference to this frame rate drop.
The second system works fine for the most parts, but can drop a little in frame rate when the AI is working overtime - large police chases (3 stars or more), + when there is a lot of explosions happening.
Something is definately amiss with this game. I have also read reports regarding people who had an older card such as a TNT2, then after upgrading to a GF2, the game suddenly runs smoothly with no lagging whatsoever (indicating that the texture optimisation is not good enough).
Also, I've read many reports about turning off HW T&L through 3D Analyse also speeds the game up quite signifcantly for some people.