Does operating a DC motor at low PWM freq (500 Hz and below) deteriorate the motor performance?
In general, you want to use the right frequency. High enough to get the motor current reasonably constant, low enough to avoid eddy current losses and PWM driver switching losses. 

'Reasonably' varies between people and applications, but 10% is probably quite reasonable, &lt;1% is unreasonably good, >50% is unreasonably bad. The frequency needed will vary with the inductance of the motor.

The two main reasons for keeping the current ripple down are (a) to keep torque variations, aka vibration, down, and (b) to keep the efficiency up. As the torque is proportional to the current, but the motor heating (losses) is proportional to current squared, any variation in current will increase the losses and so reduce the efficiency.

You don't want to use too high a frequency either, especially on a motor that has not been specifically designed for PWM control. Motors armatures are usually built from laminations, to reduce the eddy current losses due to current commutation. However, these laminations will usually be designed to a price, to be as thick as they can be for reasonable losses at the rotation speed of the motor. If you impress a varying voltage from PWM at a much higher frequency than this, the eddy current losses will be severe, and will again reduce your efficiency. A motor designed for PWM may have thinner rotor laminations, and a specification for the range of reasonable PWM frequencies.