A paddle in liquid is a simple way to load a motor. You alter it with ease, by immersing less or more of the paddle. Or by using liquids with less or more viscosity.
In engineering class we tested our homemade turbines, by clamping a slotted block of metal around the rotating shaft. We tightened the clamp until the block just started to bind. Then rpm was measured by stroboscope. This became the turbine's torque score.
Usually a motor has a certain rpm which produces the most torque, or most work delivered to a load. In other words its most efficient operating rpm. A lot of things contribute, including your controller of course.