Undervolting BLDC motor on electric bicycle
The maximum possible speed varies linearly with the battery voltage.

With a typical ESC that uses PWM to drop the voltage, and assuming the batteries are good for the current, if you want a low speed, there will be no difference in maximum torque and power. Efficiency will only be affected slightly, as a function of the ESC.

Lifetime might be improved as you won't be able to reach higher speeds.