The first thing to do is to remove those draggy, knobbly tyres and fit some slicks, that’ll make the biggest difference to your overall speed.
If you can, lock-out the forks or even consider fitting some fully rigid ones. They’ll be significantly lighter and you won’t waste energy bobbing up and down.
What’s the difference between a road bike and a mountain bike?
You won’t need the massive spread of gears that a mountain bike typically has on the road so fit a smaller range cassette. This will save you a bit of weight but, more significantly, the smaller jumps between gears will allow you to maintain a more even cadence.
Look at how you can make your position on the bike more aerodynamic as an upright mountain biking posture is going to cost you a lot of speed, especially into a headwind. Fit narrower handlebars or even cut down your existing ones. You can go slightly narrower than your shoulder width but this will compromise the off-road capabilities of your bike. A longer stem should also allow you to get a bit lower but can also affect off-road handling.
I’ve seen people fit aerobars to an MTB, but you’ll probably also have to adjust saddle height, saddle fore/aft and stem length to get a decent position. Totting up the financial and potential time costs from all these tweaks, a second-hand road bike could be a better option.