I only have mobile engines, no static ones. Some Mamods won't run particularly slowly but I always run them as slowly as practicable, the others certainly run slowly. I'm not a fan of a mobile running around at scale speed of 200mph whilst throwing oil around (and more importantly, away from where it should be). I try and work engines with some load to pull, to slow them down (if there's no throttle, like most Mamods) or I'll run them on a whiff of burner. Looks much more realistic to me but each to their own.
I have run engines flat out a) to see what they're capable of or b) to maximise power - such as for running a generator.