Your main issue is muscle memory. You are used to riding a mountain bike, your body has adapted to it. You’ll find a road bike to be a much different animal, and you are likely to find that many of the fundamentals are not the same. The reach from the saddle to the bar is different, fore-aft position of the saddle is probably different, and there is a good chance your crank arm lengths are different. A main speed advantage of a road bike is a more aerodynamic riding position, and adapting to a good position comfortably takes time. It took years of riding for me to more-or-less develop a good position, good pedal stroke, and to flatten my back.