The power level set for a 393 does not directly relate to RPM. A power setting of 127 is not 127 RPM, but it is the maximum power the Cortex can send to a motor. A 393 with a standard set of gears is capable of 100 RPM, but will never reach that with a load. When you set the power to 90, you are setting it for about 70% max power, not 90 RPM. Actual RPM will be determined by battery levels, motor health and actual net load on the motor.
It is not voltage that creates heat, it is power, which is, for dc, volts times amps. Power is measured in watts, not voltage. Using 100% of the power setting will send the maximum possible power (without looking it up, something like 5 watts) to the motor, which will heat the motor faster than if are sending 70% power (roughly 90/127).
Interestingly, the V5 motors default to an RPM setting, not a power level setting. When you tell a V5 motor with a 100 RPM gear box to “go 70” you are setting it for 70 RPM, and the built-in motor control computer will do its best to get to and hold 70 RPM, no matter what the battery state is. The V5 motors aren’t just bigger, they are W A Y smarter, too.