These days performance tends to be bit complex subject, graphics options so often affect also CPU load and to know what is affecting which is not always so easy as effect can be to GPU and CPU load, so for example with gtx1060 it is possible to add graphics options so that GPU becomes limiting factor, with faster GPU like gtx1080 that change causes CPU to hit limit. Takes bit of time and effort to figure out eventually, but yes, shaders can cause CPU load too, shadows and reflections can add to both CPU and GPU load, depends a bit how those are done, but only way to find out is to experiment and monitor load on each CPU core as well as GPU power usage and core load both. I don't know if AI and CPU graphics are fighting from same thread or if they are completely separate thread, in 1st case lowering CPU load on either graphics or AI would give boost in FPS, with 2nd, it still might be somewhat similar, I'm now sure if things can be completely separated, something waiting for other to finish etc. So it could be possible that lowering visible AI car count reduces CPU load from graphics and allows more room for AI physics, that is what makes it tricky to know for sure, I have found out with other games that testing vsync on and off might give some idea while monitoring each CPU thread, but with some games thread residency is low and pretty impossible to tell anything. You probably know much better how it is with Raceroom, but thought it is good to bring up that even with separated threads it might still be hard to tell which CPU load decrease made improvement to fps.