My rule of thumb. At the same effort level, cruising speed on a road bike with drop bars is 2 mph faster than on a mtb with slicks, which in turn is 2 mph faster than the same mtb with fat knobby tires.

The mtb that I was riding when I developed this (empirical) rule was a high end hardtail with light weight and tight geometry (for a mtb), pretty aggressive fit (bars narrow and 3" lower than saddle, bar ends). A mtb such as you described would likely be slower yet.

The reasons include, I think, aero resistance/body position and rolling resistance.