I just recently bought my first roadish bike (its a cyclocross bike), I used to ride exclusively on an old front suspension mtb. I use the mtb as my commuter and the cross bike as my fun ride. My average pace on the mtb is 15-16mph. Just recently I rode the same route with my cross bike just to see how big of a difference it makes and here is what I found. On the cross bike my average pace is 20mph and I work less to maintain my pace than I do on the mtb. Here are a few of the likely reasons for the increase. The mtb's front suspension absorbs some of the power from the pedals making it less efficient but more comfy. The mtb has smaller wheels 26x1.75 vs the cross bikes 700x33. The cross bike is lighter as well, it weighs 22lbs and the mtb easily weighs 30+. And the last reason I think my cross bike is so much faster is that it has clipless pedals and the mtb uses flats. SO in conclusion how much speed gain you will have switching from your mtb to a road bike depends on a lot of factors.