Typically single speed pumps are too big for your pool. Dual speed and variable speed pumps address this wasted energy as they can operate at different speeds. Running your pump at lower speeds for longer periods of time will cost you much less in operational costs than running your pump at a high speed for shorter periods of time.
The energy savings that are gained by replacing a single speed pump with either a dual speed or variable speed pump are based on an engineering principle known as the Pump Affinity Law. This principle states that the energy savings are disproportionately larger as you lower the speed of your pump.