Riding fast on an MTB
The critical factors are rolling resistance, wind resistance, gearing, and body posture.

Rolling resistance is minimized with smooth, high-pressure tires.  It's not required that they be super-narrow, but narrower than 2" is probably best for wind resistance.  You should generally run 80-100 PSI.

Wind resistance is minimized by a head-down posture, with arms fairly close to the body.  Changing out the handlebar, or installing bar extensions, may be necessary to optimize this.

Gearing is important if you "max out" your high gear, or you can't find a gear to let you run at optimal cadence.

Body posture (over and above issues of wind resistance) involves having the body "fit" the bike in a way that lets you pedal efficiently at high cadence (and with your head down).  This is more a function of the bike than the other factors, and likely harder to optimize on a mountain bike.

(Remember, maintaining speed on the road is not a matter of putting out maximal force, but rather sustaining power output over a long period of time.  Higher cadence with less force is generally better than maximizing force.  A cadence of 80-100 RPM is generally considered optimal, and a reasonably good first take at balancing force and cadence is to be doing two revs of the crank for every breath you take.)

Oh, and lock out the suspension.