Also be careful when breaking, as the motor controller will inject the energy back into the power supply which can blow up the controller, unless there somewhere for the energy to go to. If the energy can't be stored in a capacitor or battery, it will need to be dissipated in a resistor.
Another thing which can help increase the response time is to use a higher supply voltage, than the motor can continuously take, which won't cause any harm, unless it's applied for long periods of time or is excessive, causing the commutator to arc over.