That’s really what it boils down to…a Honda Shadow 650 (V-Twin) or a Suzuki S40 (650 single) will get to 80mph (130kph) but it doesn’t happen fast…a Ninja 650R (Twin) will do it pretty quick and keep going…a Suzuki GSX650F (inline 4) will do it even quicker and keep going farther…then you get into the CBR/GSX-R/R6 type bikes (race inline 4) they will do those speeds in second gear, maybe even 1st…and they have less engine to work with…compression of the engine, restriction in the air intake and exhaust, design of the heads, grind of the cams, weight of the crank/piston/con rod assy…that will apply to 250cc bikes just as easily as a liter bike…
I know the guys in here all harp on about 250cc being the ideal bike to start on, but even at that there can be big differences…a Honda Rebel vs a Suzuki DR200 vs a Kawasaki Ninja 250…the cruiser would be the most beginner friendly…low seat, low power, low speed…the DR200 would be a step up (no pun intended)…a higher seat height, similar power but the gearing is lower so it will seem to have more, and it will get to top speed quicker (but probably similar actual top speed…fastest I could get one going is 55mph-90kph)…then the Ninja 250…it would seem like a race bike in comparison with the other two…seat height in between the two, twice the power and almost twice the speed…so in reality…a 650cc cruiser with mild tune is comparible to a 400cc dual sport which is comparible to a Ninja 250…and all this is assuming you are average height, build/weight…
There are so many variables…there is no real “right” answer…since the answer will be different for almost every person.
Later.