I could be WAAAAYYY off the mark here as I'm not a scientist / physicist but I would have thought as you're pedaling you generate a certain amount of energy which needs to be displaced somewhere ... if you have soft forks they'll absorb some of that energy in the down up movement rather than it all maintaining the forward momentum / direction of travel ... as I say I have nothing to back this up with but to me seems reasonably logical.
The only thing I can say from experience is it seems faster pedaling a MTB on road / cycle path with the forks locked out to the extent where we do a mix of gravel track / road I always leave the suspension locked as it feels more sluggish when it's not ... but that's obviously not very scientific either
If I remember I'll ask a rocket scientist in the making tomorrow as he may possibly have some constructive input about the displacement of energy.