Just came here to address a few issues, that I am currently working on, as mentioned by TooManySugar, the carfx and surface manager, some how slowed things down after the release. I probably made a few mistakes before releasing. It seems like the new versions of Unity are really bad at material handling (visual / mesh surface) Because at the time I did my first 100 car test I had the surface manager working, then I updated to Unity 5.6 and did not stress test the product. So at the moment I am working on a alternate route to handle materials. because if I scrap the materials, I can run 64 high poly AI cars on a well detailed track at stable 60 fps, even with atmospheric scattering and other GPU and CPU demanding VFX enabled. I will report back once I get a stable solution.