Why is PWM speed control better than dropping the voltage?
Instead of cutting the voltage, PWM sets the average power by switching the full voltage on and off quickly. The heat loss on the driver is far lower and the motor keeps its torque at low speed.
Dropping the voltage with a resistor turns the excess energy into heat: efficiency falls and the driver heats up. PWM (pulse width modulation), on the other hand, switches the full voltage on and off thousands of times per second; the average voltage the motor sees is set by the pulse width.
The result: the loss in the driver stays at a minimum, and the motor largely keeps its torque even at low speeds.
Full topic: DC Motor Driver Selection Guide