What is optimum in your context?
For optimum efficiency, run the motor as fast as possible, which means low torque loading and high voltage, and adapt the torque requirement at the load to the motor's ideal torque using gearing. Run the motor fast and gear down.
Look at the motor's datasheet and note peak efficiency is somewhere close to the unloaded speed. (This will also show that if you get TOO close to the unloaded speed, like 90%, efficiency reduces again in a brushed motor, because the friction of the brushes steals torque from the output)
This ties in with Marko's answer in that waste heat comes from current, and gearing to reduce torque reduces current.