If you think that because you have lots of cars around you your video card can't handdle them and that's why your FPS drop you might be right, only if you have a really old video card like a GF440, otherwise the main FPS drawback is from CPU and belive it or not it's from the sound engine, try to disable your sound card and then run LFS, you'll see that you get a lot more of FPS, this applies to the start grid when all the cars around you are making varous sounds killing your CPU.
Another way to test that is to watch a replay in 2x, that way LFS automatically disables sound, you'll see a sudden increase of FPS.