Yes, maintaining proper speed is important with a pto generator. As pointed out above, the e-pto concept is best when the tractor is capable of a lot more power than the job needs.
If a 30 hp tractor is connected to a 15kw generator, the 540 pto speed (engine at 2400) should be used - so the governor is responsive and rpm is stable with changes in load. But the same generator with a 60 hp tractor and 750 pto, the engine can loaf along at around 1700 and the speed will be stable. (my experience)
Running a generator with the e-pto there isn't much difference in fuel consumption - power delivered is power delivered, independent of rpm. At lower engine speed there is less friction, less wear, and less noise, but the fuel saved running the same load is small if anything. If you look at a curve for the typical diesel engine, there isn't much difference in efficiency between 1700 and 2400 rpm.