I don't think it could be something subtle, like tire pressure. Too much difference for that - you'd practically have to have a flat tire for the difference to be this big.

The main things I can think of are extra sources of friction (e.g. parking brake not fully disengaged), a defect in the engine timing, or a defect in the batteries or their charging circuit. The problem with all of those ideas is that I would expect a "check engine" light or some other obvious warning.