Hmm.. you may want to make a build of the game just for testing- try it on different hardware. Perhaps you could code a FPS counter (for testing purposes) and people tell you what they get?

With Blender it tells me the percentage of system resources each part is using (which is handy!) so you may have a job seeing what is the bottleneck (if any). I'm guessing Unity requires a strong video card.

On other news, just finished my own node navigation script! It allows me to use any number of nodes in a track and will automatically sort the racers into race order....I can finally rest easy (I get annoyed with myself if these problems linger too long!). Maybe I will get to finish my game too