If a condenser is speeded up, airflow will increase and the condenser temperature will decrease, making the system more efficient (ignoring the power used by the condenser fan). But at some point, the energy used by the fan motor will undo the savings from lowered condenser temperature.
Is there a "fast and simple" way to calculate the optimum fan speed or does it vary too much depending on the design? (This is all just for curiousity, BTW.)