If you just purchased the bike specific to the mixed terrain....but seem to find yourself on the road more I could make no suggestion other than the dedicated road bike.
In the case that money or SO support isn't into that one, a second set of wheels and narrow tires will help a LOT in respect to those 47mm boat anchors. Even if you went down to something in the 28+ <40mm range it would make a huge difference in your rolling speed.
The other alternative here would be to enjoy the purchase you made for the reasons you made it. Enjoy the ride and worry a whole lot less about average speed. I would probably consider this route the hardest.