I have those same gears and have done a few MTB races that have long road leadouts (1-2 miles). I can hit 25mph+ with those gears but not for a sustained ride. The gears are not your problem. I would guess it is your cadence.
The big problem will be in the rolling resistance of the tires and the wind resistance from the frame and position it puts you in.
You can put money switching those things around but then you get a bike that will do both trails and road but will do neither very well. Your best bet is two bikes or find what you like best and have the bike built for that and just deal with the other aspect with what you have.