I suppose in the end, it would depend on the load on the motor. You certainly see more heat in the resistor box when the resistors are in the circuit but the overall current will drop as you add resistance.
If you are pumping a fluid (gas or liquid) the load itself decreases as the rotor speed drops. If you were attached to a direct mechanical load you would start approaching locked rotor a lot faster and that would be the effect you are talking about.
DC motors are not going to react to voltage decreases like an (AC) induction motor tho.
