Commuting on pavement? Then you will shed of speed on an MTB. It is a combination of the extra weight, the energy-robbing suspension bob, the ridiculously wide Q-factor of MTB cranksets, the extra wind resistance and most of all the heavy wheels and knobby tires.

You could recover some of this inefficiency by changing over the tires to something as light, high pressure and as narrow as possible. Slicks of course. Since the MTB wheels are way overbuilt for road use, using some lighter, cross type wheels would be the second biggest upgrade. Then ditch the unnessecary suspension fork for something rigid. This will remove the suspension bob, and tighten up the steering. Then lose the heavy, unncessary disk brakes. Then add some drop bars so that you have more hand positions. My wrists and hands go dead after a few miles while riding on flat bars on pavement. On anything but short convenience store rides, drop bars are more comfortable and far more efficient.

Next upgrade would be to change out the overbuilt frame of the MTB to something lighter and with a narrower Q-factor.