It is true that it will be more efficient at lower speed. Most 2 speed pumps move 1/2 the water volume using 1/4 of the power on low vs high speeds. The same curve applies to VS pumps although I don't know the ratio. Gallons pumped per watt will be higher at lower pump speeds.
My VS pump moves plenty of water for skimming and making chlorine at 1100 rpm and only uses 150 watts of electricity. For solar it runs at 1950 rpm and moves substantially more water and uses 550 watts. At 2500 rpm is uses 1150 watts. It does not seem like it pumps twice as much water at 2500 rpm as it does at 1950 rpm even though it is using twice as much electricity.
Yes, I do suppose if you had a flow meter you could set up a spreadsheet to calculate the most gallons pumped per watt to determine the most efficient pump speed. But, that seems like a lot of effort when that flow rate might not be right for your application. And gut feel and low flow sensors for heaters, SWGs, etc will get you close enough. For solar you want your panels to be relatively cool to the touch and the water coming in to the pool not to be too warm. The cooler the panels are to the touch the more heat the water is transferring and taking to the pool. This is more important than a certain gallon per minute flow rate or a certain pump efficiency.
