My cruising speed on flat terrain on a road bike is around 21mph. On my bionx bike turned off, it's around 20mph.

On a particular downhill, I can coast up to 33 mph on my road bike. With the bionx bike turned off, I can coast up to about 31 mph on the same downhill.

So the motor is not really that much of drag.

Also, my bionx bike also has wider lower pressure tires (bionx - 32mm 80 psi, road - 23mm 120 psi), so I don't know how much of the additional drag is from the tires.

Charging the bionx while riding on flats or downhills may sound inefficient energy wise -- the energy is going from your foot to the battery -> motor -> wheel instead of foot directly to wheel -- but at least you can avoid riding with a lot of strain. (more aerobic leg work and less anaerobic).