That is somewhat of a generalization, often based on the fact that a 2-stroke of any given displacement, will rev higher than a 4-stroke.

However, there are plenty of *very* fast 4-stroke-powered models which rival or exceed the speed produced by similarly powerful 2-stroke engines, particularly when it comes to scale(ish) aircraft.

My P51 with a 4-stroke 91 is slightly faster than a friends exact-same model with a 2-stroke 90 engine.

The difference is that he's running a 13x8 prop and I'm running a 12x10.

He pulls about 12,000 RPMs and I pull about 10,500 or so -- which means our top-speeds are pretty damned close. Of course mine sounds liike a tractor and his sounds like a demented blow-fly. We each like the respective sounds of our engines however, and the realityh is that both planes fly just fine.

Of course I also get almost twice the flight-time from a tank of fuel (gloat).