Hi Danielsmusic,
The paralleled transistors in your circuit act like a resistor in series with the motor. When the motor is loaded, its current requirement will increase. But the "resistor" in series with it will have more voltage drop across it when the motor tries to draw more current, resulting in less voltage (and current) for the motor and therefore a loss of torque.
Pulse-Width-Modulation is used in wheelchairs and golfcarts and applies full power to the motor in pulses at a very high rate. The duty-cycle of the duration of the pulses compared with the duration of the off-time determines the speed of the motor. A tachometer can also be used to vary the width of the pulses so that the speed of the motor remains constant with a changing load, or when starting.
Since the transistors fully switch on and off, they don't heat nearly as much as transistors used as a resistor. :lol:
Win-win. Lotsa torque and not much heating.