The CEC measurement data actually shows that most two speed pumps use about 1/4 the power on low speed than high. Some are better some are worse but on average about 25%.

However, I think there is some confusion from the previous posts on the application of pump affinity laws. These are stated as follows:

GPM Low Speed = GPM High Speed * (RPM Low Speed / RPM Hi Speed)

Head Low Speed = Head High Speed * (RPM Low Speed / RPM Hi Speed)^2

HHP Low Speed = HHP High Speed * (RPM Low Speed / RPM Hi Speed)^3

HHP, the hydraulic HP, is what goes down by a factor of 8 but the pump affinity equations do not account for efficiency in the pump and motor so adjustments are required to calculate the change in input power. 1/2 speed on a typical pump tends to be about 50% less efficient than high speed because twice the windings and fields are involved. So the actual power draw of low speed is about a 1/4 of high speed for most pumps. Some pumps are better some are worse but again, you can see this in the CEC measurement data.

Also, you can explain the difference in pressure by using the head loss formula so the pressure should decrease by a factor of four. However, you need to take into account the height of the filter gauge so the adjustment needs to be:

(18 PSI + 3'/2.31) / 4 - 3'/2.31) = 3.5 PSI

Gauge accuracy is no better than +- 1 PSI and some even worse so it is pretty close to what is observed.