Agreed, but therein lies the fundamental difference between internal combustion engines and electric motors. Gas engines need to operate at a minimum rpm to make any useful power, and must fire in fast enough intervals so it doesn't shake itself to pieces think of a lawn mower about to stall.
To get a variable pumping capacity wouldn't an unloader to the job easier?
Idle speed is enough to prevent the motor shaking like crazy. At idle the engine horsepower will be minimal. Electric motors don't operate efficiently at less than about 40%, I think the gas engine has the advantage in part load capacity. Underloaders would be more complicated to implement that just varying the throttle position of an engine. Why produce the power then unload it?