How to find the optimal speed of a BLDC motor having hall sensors? Can we change the most efficient speed through the controller?
The maximum efficiency of all electromagnetic motors occurs at or near the maximum design speed. The efficiency variation with speed is lowest at high speeds as the speed approaches zero the efficiency declines more rapidly until it reaches zero at zero speed.

If a machine must operate at a speed below any available motor speed, it is generally best to select a motor with the lowest available maximum design speed. Not much can be done with the motor or controller design to make the system more efficient at reduced speeds.

Electrostatic motors are most efficient at low speeds. The efficiency is still zero at zero speed, but they can produce high stall torque using very little power. However, there are virtually no such motors on the market today.