Let's say in 99.99% of cases the point is keeping the motor in a safe range of operating temperature. You're not going to notice any performance difference unless chasing that one or two MPH more. So just forget cooling for performance. For most of us, it's cooling so the motor doesn't die of...
Personally I don't run fans on motors in my Slash 4x4 and Kraton 6s, I'll rather gear down if I find the motors are cooking. Or go for a bigger/better motor if that's what's needed. I'm having enough trouble with ESC fans dying on me and I hate additional wiring and stuff just waiting to fail 🙄