It's really common on a mountain bike to run a tire in front that is very focused on traction and a tire in the rear that is compromised on traction to improve rolling resistance. The front does all the steering and braking and the rear carries most of the weight. This can be taken seemingly too far sometimes (like Specialized Butcher/Slaughter).

But that's only if you want to keep it a Mountain Bike.

If you want to mostly ride roads, go ahead and get slicks, they will go a lot faster. People have been turning mountain bikes into commuter bikes for 35 years. Lighter ones are faster and smoother, but that's because they don't have as much armor, so pick your poison. As long as the tire is more than about 50% wider than your rim, you are ok.