All Single Speed Pumps run at 3450 RPM. Most pool pumps are single speed. Most pools use single speed pumps with the horse power (HP) between ½ to 2 HP. Running a Pool pump can be costly, that is why manufacturers designed a pump that uses much less electricity to keep your pool crystal clear. As the name suggests, a Variable Speed Pump can run at different RPMs.
If you cut the RPM on your pump to 1/2, for example, if you go from 2000 RPM to 1000 RPM, your electricity usage will be much less than half as much. That is why Variable speed pumps make sense. You can filter the same amount of water, for a fraction of the cost.
At the present time some Power companies offer a rebate to homeowners, if they upgrade to a Variable Speed Pump. DWP offers a $1000 rebate, and Edison offers a $200 rebate.