I'm no competitive cyclist, and in pedal assit mode on my eBike, the motor stops being because I'm pedaling at 20mph on my own. My eBike is also a big heavy bike, so from where I'm pedaling, I'd assume others can do the same on a better bike. <shrugs>

Power required increases with the square of the speed being primarily a function of wind resistance at higher speeds. Your motor shutting off cause you hit 20mph on a slight decline might give you the wrong impression.

Maintaining 25mph requires in excess of 300 watts. Some 90% of the population won't be able to produce that for a mere minute.