The controller/thermostat on a stabilized freezer turns on and off in a steady state as it reaches setpoint and then drifts away from it when the motor is off.

The controller/thermostat on a stabilized freezer turns on and off in a steady state as it reaches setpoint and then drifts away from it when the motor is off. Filling your ice cube trays with hot water forces the thermostat on sooner, makes it stay on longer because of hysteresis in the controller settings and so freezes faster. That is the only demonstrable reason I have ever seen.