In my own pool when switching from a 1 HP (full-rated; 1.65 SF) single-speed pump and 3/4 HP booster pump for the pressure-side pool cleaner to an Intelliflo VF pump for both, my annual electricity cost for the pump(s) was cut by around 50% from $1400 per year to $700 per year. I have a solar system which is why I didn't get more savings since the solar system needs (well, is recommended at) 4 GPM per panel. I could probably save more by going to 3 GPM per panel which is the minimum, but I'm giving a more apples-to-apples comparison as the 1 HP pump was giving at least 5 GPM per panel.
If I didn't have a solar system and could run the pump at a low speed (26 GPM) for one turnover per day, then my annual cost would be around $270 per year so a roughly 80% savings. Getting a 90% savings would have to be a rather extreme example of an oversized pump or bad plumbing (i.e. 1.5" lines). Nevertheless, 80% is pretty significant. Of course, if one compares to a 2-speed pump run at low speed, the savings isn't nearly as dramatic, meaning that the 2-speed pump gets you a lot of the way there in savings though will be more expensive since it's less efficient as the Intelliflo at the lower speeds.
Though the power consumption at very low flow rates drops off, if you factor in the longer time needed for the same number of turnovers it turns out that the Intelliflo operates at around the same cost, at least in my pool, from it's lowest 15 GPM (18 hours at 155 Watts) to 26 GPM (10 hours at 275 Watts). It's only above 26 GPM (for my pool) where the daily power usage climbs.
Richard