RPM is linear with flow rate. So 2xRPM is 2xGPM.
Thanks for straightening that out, Mark. I know it doesn't work that way for a boat, but I didn't mean to mislead for a pool. In both cases, I was considering the entire system, not just the "engine." That said, my [admittedly uneducated] instinct about it: I am only slightly surprised to learn that about a pump, alone. But I can't help but wonder if that remains true if you consider the pump connected to a pool plumbing system and moving water into and out of a pool (which I wasn't clear about). I'd be very surprised if 2xRPM (and so 2xGPM) is actually moving 2 times the water in/out of the pool. Maybe at the low end of the graph, but at some point the efficiency must taper off.
Back to the boat analogy, I could be persuaded that 2xRPM on the prop is moving 2 times the water past it, but that boat is not going to be moving twice as fast. Drag, I suspect, is the contributing culprit. Certainly that must be true of a pool plumbing system. The harder that pump pushes, the less efficient the water exchange must be.
No matter, the over-arching point is that the OP is going to see lower electricity costs and/or improved equipment performance by using a variable speed pump. It
will pay for itself over time (and I still contend it offers other advantages too). Perhaps it really boils down to
how much time. I'm projecting my own purchasing decisions on this thread, which are based on the fact that this is my "forever home." So no matter how long it takes to achieve ROI, I can wait it out. But if the OP knows that his stay in his home is a limited number of years, then the span of time for ROI becomes more important to consider.
There, did I weasel my way out of that sufficiently?