Driving alone in single player I can get 125 fps if "limit framerate" is [off]. But if I set limit to 100 fps, LFS makes only 92. Set limit to 90 fps, and get 84, and so on downto 50 fps when the limit and the result meet.

Is it just fps limiter that limits the performance more? Or, maybe, when the limiter is [on], some action additioally consumes GPU/CPU resourses?

i bet its your ms latency settings
go to options - misc - minimum sleep, and say what number that is
if its over 1, it will lower your FPS; but by doing so, your giving the computer that many more milliseconds to compute

Oh, yes, forgot about that. My minimum sleep setting is 1, but I haven't noticed any difference when changing it.

Athlon XP 2600 (don't remember the "name", it was the last version of 2600), GeForce FX5200 128mb/128b.

I've also noted a significant FPS increase (in my usual settings 72 => 120 fps) after I changed view from [incar] to [custom]/[nothing].