As a follow-up.... I discovered that the pump speed of the IntelliFlo (IC20) needed to be set at >1,500 rpm in order for the flow light on the IntelliChlor to be "green". I thought that the speed was a bit high, but maybe that's characteristic of the IC20 as well as the length & diameter of the underground PVC plumbing.
Since the IC20 has fewer plates than the 40 and 60, it is unlikely that the IC20 is the reason you need to be up at 1500. If anything, the IC20 would allow a "slight" lowering of RPM because it would have slightly less friction.
Recently, I replaced my very old 48 sq ft DE filter with an 80 sq ft DE filter, and replaced the higher friction plumbing [lots of elbows], and higher friction slider valve, with a high-flow valve, and lower friction plumbing with sweeps instead of elbows wherever I could. My max head pressure at the filter went from 40 psi to 20 psi. No surprise, my minimum "safe" RMP speed went from 1800 RPM to 1100 RPM.
So basically, it is the flow friction, also called head pressure that the pump us pushing against. While the IC will contribute to that it is a tiny portion. Your filter and elbows/plumbing are the lions share.
So if you want to drip your RPMs, look at higher surface area filter, and higher flow valves/plumbing. Finally, I forget who gave us this information, but the Intelliflo loses efficiency much below 1000 RPM, so there is little motivation to go lower.
To give you some info, going from 1800 to 1200 RPM on my system gets me from 415 watts to 164. So my power requirement was reduced by half. Pretty cool. At 1600, I'm at 317.