Any properly designed PWM system, even a single ended one-transistor type, can be very efficient power-wise, but the speed is likely to vary with changes in load to some extent, without feedback control.
The efficiency depends largely on the switching speed of the power device(s).
The stages of improvement are firstly speed feedback, then in high power applications where speed accuracy is important, a full-bridge PWM control with current and speed feedback is commonly used.
The current feedback loop compensates for the torque demand and the speed feedback loop adjusts the current to maintain accurate speed.