Road bike vs. road bike, probably not so much as Wilfred said.
Comparing different bike types? Well, I can say that my aluminum mtn bike at some 20 lbs takes substantially more effort on flat road to keep any kind of speed up. Just staying at ~13 mph is a constant chore where my aluminum road bikes I can cruise at 15 no problem.
IMO, bike type / weight, wind, aerodynamics, tire type, and rider position (sitting mostly up on the mtn versus down on a road bike) all factor in my ability to ride a sustained speed.