i don't know the exact percentage, but it is true road bikes have less resistance. I would get you lower it by 10% just switching to road tires on a mountain bike. road bikes weigh less from the start, all the components are lighter since they don't have to be built to take the pounding from the trails, and the tires have less rolling resistance being lighter and narrow. If you only ride on the road, a road bike is the way to go.
I ride off-road and don't own a road bike. I would buy one if i road on road more than 10 times per year.