At some point, you'll run into the "fixed" energy used by the pump regardless of speed. I have an IntelliFlo VF pump, but I've tried different manual speeds and found the following where you can see that the lowest speed isn't necessarily that much more efficient when accounting for the time of one turnover. There is a flat spot near the slower speeds and the ideal may be anywhere in the 15-26 GPM range, though obviously this depends a lot on the specific plumbing. The runtime for one turnover is for my 16,000 gallon pool.
15 GPM (17.78 hours), 1185 RPM, 155 Watts ---> 2.76 KWh
18 GPM (14.81 hours), 1325 RPM, 195 Watts ---> 2.89 KWh
24 GPM (11.11 hours), 1460 RPM, 255 Watts ---> 2.83 KWh
26 GPM (10.26 hours), 1495 RPM, 275 Watts ---> 2.82 KWh (measured at different time so might be a little higher in reality)
30 GPM (8.89 hours), 1805 RPM, 420 Watts ---> 3.73 KWh
36 GPM (7.41 hours), 1975 RPM, 535 Watts ---> 3.96 KWh
40 GPM (6.67 hours), 2075 RPM, 615 Watts ---> 4.10 KWh
42 GPM (6.35 hours), 2160 RPM, 685 Watts ---> 4.35 KWh
48 GPM (5.56 hours), 2310 RPM, 835 Watts ---> 4.64 KWh
54 GPM (4.94 hours), 2490 RPM, 1030 W ---> 5.09 KWh
60 GPM (4.44 hours), 2710 RPM, 1305 W ---> 5.79 KWh
66 GPM (4.04 hours), 2900 RPM, 1590 W ---> 6.42 KWh
72 GPM (3.70 hours), 3080 RPM, 1895 W ---> 7.01 KWh
78 GPM (3.42 hours), 3360 RPM, 2495 W ---> 8.53 KWh
I do notice that surface circulation and skimmer vortex suction isn't as efficient at lower speeds, so Jason has a good suggestion of doing a mix so that you can still get decent cleaning. Since I have a pool cover that tends to inhibit surface circulation anyway, I just have my non-solar flow set at 26 GPM. Usually, the solar is on 4-6 hours a day with its 48 GPM and the total cycle is 8 hours. The solar energy cost is far higher than shown above since the 48 GPM requires 2980 RPM and 1500 Watts.
Richard