How does riding on bumpy road while suspension fork locked and without slow down effects?
On a bumpy but fairly level road, absent any suspension, energy goes three places:


Friction between tire and road
Wind resistance
Vibrating your body up and down


The reason #3 is a factor is that you jiggle as you ride, and that absorbs a lot of energy -- the more you jiggle, the more energy is required.

You can reduce jiggle by using lower air pressure in the tires, but this increases tire/road friction.  You can add front and/or rear suspension to reduce jiggle, but the suspension itself absorbs energy, plus it tends to make pedaling less efficient.

So it's a tradeoff -- at some point in the transition between smooth and really rough roads it probably pays, in terms of energy efficiency, to unlock the shocks.