The fan speed control switch only controls the speed of the fan while the compressor's speed varies with the rpm of the engine and is not variable through some controller. However, the fan speed does alter the rate at which the compressor turns on and off, frequently turning on and off at low speed while staying on for longer at high speed. This happens because of the thermostat installed in the evaporator unit. At low speed, the coolness of the ac stays longer within the unit, making the thermostat think that it has reached the cut off temperature while at higher speed, it takes longer for the unit itself to reach the cut off temperature because the coolness is transferred to the cabin.