This was a good read I found helpful - a May 2012 publication from the U.S. Department of Energy:

http://www.nrel.gov/docs/fy12osti/54242.pdf

It describes the energy savings of variable-speed pumps, and how best to operate them. I see now how important the skimmer is to my pool! It's much more than just getting leaves, grass blades and big bugs off the top of the pool's surface.