A little bit of background on how motor controllers work. Ideally we would vary the voltage going to the motors to control them. We don't do this because the circuitry to do so would be overly complex. Instead we do something called PWM(Pulse Width Modulation). In pwm the controller can only switch the motors to full supply voltage or 0 volts. We acheive the effect of 'part throttle' by switching the motors on and off very rapidly. The percentage of the time that they are on is called the duty cycle and is usually measured in percent. The number of times we switch them on and off per second is the pwm frequency or as you referred to it 'chop rate'. The motor has a high inductance so it smooths out the signal.
The wave forms look something like this if you plot time on the x axis and voltage on the Y. In the graphs shown below the duty cycle varies but the pwm frequency is constant.
Code:
0% duty cycle
|____________
25% duty cycle
|-___-___-___
50% duty cycle
|--__--__--__
75% duty cycle
|---_---_---_
100% duty cycle
|------------
The pwm frequency effects a number of things including system dynamics, energy waste, motor insulation and audible as well as emi emissions. The pwm frequency is usually in the audible range of human hearing which results in whining noises. The industrial motor controllers i deal with a work have a default pwm frequency of 4khz. Someone is playing with a motor a few cubicals down as i write this and it is rather annoying.
More importantly PWM frequency effects heating. If your PWM frequency is too high, you will unnecessarily waste power heating up your motor controller as it switches on and off. If the frequency is too low, you can heat up the motor. If you went to an extremely low frequency your system would start jerking, becoming nonlinear and generally behaving strangely.