You generally want to run a motor at 66-75% of its free speed, which will be 25-33% of it stall torque and stall current. If you look at
Motor Information you can see what speed and torque this will be for a CIM.
It is good to avoid running the motor slower than 50% of its free running speed as you can get the same amount of power out of the motor by changing your gear ratio and running the motor faster, producing less heat and using less energy.