Hi Mawgly, thanks for the feedback and I'm pleased it is mainly working well.
The AI drivers can use a lot of CPU power for their physics, and a lot of GPU power to draw them, so either of these could slow down the frame rate. It's hard to know which is the main problem, but if you could tell us details of your CPU and graphics card then maybe someone would have an idea. If the CPU is the problem, you may have better results going online. Your computer does not need to process full detail physics for all the remote drivers when you are online. So if your CPU is limited, that may help. If you press "P" (in single player) to pause LFS and the frame rate is still low, that suggests your graphics card can't handle drawing so many AI cars. Some people should upgrade their graphics card to help with the Rift, and it may not be very expensive compared with the price of a Rift.