I have hit 55kmh on the flats with my 20 inch wheeled folder... this is where I max out the 53/11 and can't spin any faster. My pleasant cruising speed tends to be in the 30kmh / 18mph range and that applies to pretty much all my bicycles.

The wheel size has much less to do with things than does conditioning and aerodynamics, my folder and smaller wheeled moulton both have drop bars and fit like a conventional road bike,