As long as you can get "aero", there is no big speed difference between bicycle types running close to the same tire pressures, even when running different tire types and sizes. What makes any bike fast is the strength of the rider and aerodynamics, everything else is very minor. I tested my time over a 12+ mile route with no steep grades on three different road bikes and a classic MTB with 2.1" wide knobby tires, and I have finished the same route within 20 seconds on all of the bikes in the last year. The only change on the MTB setup between riding it offroad and on pavement is running either 40psi or 80psi in it's tires. So as long as the bike is sized to let you get horizontal, there is no advantage in speed in any other bike. It IS more comfortable to ride the road-bikes in a horizontal position though, but that is because of the drop bars of course. Here are two of my road bikes and what my MTB currently looks like;
