With the development of highly efficient variable speed pool pumps, two-speed pumps have largely been superseded.
Two speed pumps do offer considerable cost savings for the pool owner. In low speed, the pump runs at half speed (1725 RPM) and uses only 1/4 of the electricity. Over the course of a year, this can save the pool owner a LOT of money.
See our variable speed pump page for more information on the features and benefits of variable speed pumps.