On the flats aerodynamics makes as much difference as weight, perhaps more. When I ride a fairy fast MTB commuter upright body position, getting +15 means real work. Switching to my CF road bike I'm at 19mph working no harder. Beside my road bike being much more aero my bars are 2 inches below the saddle, I'm showing a lot less body to the wind. There are other variables for sure, like using different muscles in a rotated position, but if you get a road bike that is set up so you're sitting upright...it's still going to leave you like a sail in the wind. If you go up hills at any speed, same effect. You may have to work on your riding position if you want to go faster, regardless of the bike you're riding.