Hi, thanks for reporting,
yes I tested with 4 AI and it lags a bit, this could be optimized for example, wheel suspension raycast set extra frames to skip, or AI reactions reduce to fewer frames.
But the main "problem" here I believe is because this asset was made for standalone version and later I changed to mobile shaders + controls.
Building a mobile game I would use more simple 3d models, models in the asset are with doors, trunk, hood, functional windows, interior to show all the possible features you can use.
Another heavy thing is that all objects are loaded in one scene, I am working on the Unity 5 edition, where you load menu, then choose car and later load the actual race scene, this will optimize recource loading too.
AI is planned to improve to work better with mobile.