Not so much the bearings creating heat at high RPMs, but the motor.
There are two major types of losses in motors. Copper (from running high current through the wires), and Iron (from magnets moving past the iron core).
Copper losses are generally at low RPMs, and iron losses at high RPMs.
There is a sweet spot where the losses are at their lowest in that trough on the gray line.
It’s funny that the sweet spot lands right at 190kv on 12s.
Selecting kv is all about battery voltage.