I've used removable pedals including MKS and others on all my folding bikes for the last 15 years or more. I think there will be zero energy loss due to the play in the bearings/spindlehousing. The weight of your leg alone is enough to keep the pedal in the " sagged down" position. It doesn't require any muscular input from the rider to keep it in that position. In addition, the pedal remains permanently in the sag down position whilst pedalling. You may imagine that the pedal waggles up and down as you pedal, and if it did there might be some loss, but that isn't the case.
Edit: The above assumes your foot is not clipped or strapped to the pedal. If it was, and if you also pull up on the pedal on the upstroke of the crank, then the pedal spindle WILL waggle up and down during rotation and that may generate a small energy loss.