Edit: My fan had gathered a lot of dust and I guess running at the lowest frequency prevented forced throttling due to overheating. This tip may thus not actually help with fps problems.

I had big problems with the game being very slow in some areas. (Note: I play on Linux using wine.)

I have a laptop and the CPU frequency changes by default depending on usage. When I forced a fixed frequency (800 MHz to keep the fan down) my frame rate problems disappeared and I can play the game just fine, as I would expect with a PC so much faster than the PCs of the period when the game was released.

Most PCs these days - even Desktop PCs - use some form of frequency scaling to save power on idle and this might be a reason for bad performance in the game. Try setting the power profile to "power save" or "high performance" to force your CPU to a specific frequency and see if this solves your problem.