[Al Skierkiewicz]

The brush position has been beaten to death so far (all of it true)but there is something else that is a factor and that is the brush/commutator switching frequency vs. the speed controller switching frequency. This varies for motor types as the design of the commutator is optimized during the motor design phase. At some point the (high RPM) the two frequencies interfere with each other and the motor efficiency drops off. Also keep in mind that the difference in speed is greater at higher RPM. If you design the drive train and running speed to keep the motor away from running full RPM, you will minimize the effect. There is no substitute for practice, lot's of practice!