To add to the explanation above, motor torque is directly proportional to the motor current (VexOS actually reports the torque based on the motor current measurement). Motor RPM is somewhat proportional to the motor voltage - the actual motor voltage is the back-EMF (directly proportional to the RPM) plus the resistive losses on the motor windings (which are current, thus load/torque dependent).
Efficiency, as mentioned, is power out (torque * rps * PI) divided by power in (voltage times current). Now, Efficiency of a DC motor is a tricky beast - it represents the effect of the above resistive losses (and to some extent mechanical friction losses, which, again, VexOS estimates for the torque measurement based on voltage, thus speed).
Now, if your motor is stalled (max torque times no RPM), it performs no work, thus has zero efficiency, only heating the winding with the input power.
If the motor spins freely (zero torque at the max RPM), it performs no work either and thus has zero efficiency again (uses quite low current to fight the friction).
Bare idealized DC motor would produce the highest power at about half the stall torque and half the free RPM. At that point, it would be about 50% efficient (half of the input power moves things, half heats up the winding). At lower loads, you can attain higher efficiency, but at very low load, the friction losses of real motor would overtake the curve again.
An important note is that V5 Motor is not a bare DC motor. The nominal stall torque (2.1Nm for the 100RPM cartridge) is a software limited stall torque (the internal motor would produce significantly higher stall torque with the V5 system voltage) and the free speed is software limited too (it would spin at least 30% faster on a full battery, if not limited by the controller). Because of that, both the max power and efficiency curves are very different from bare motor.