I recently borrowed a road bike off my brother-in-law. Just recently I have been racking up 50 miles a week on a Zesty, so I was keen to see how I would cope with a road bike and crazy road gearing (no compact on this bike).
My first ride with 1200 ft climbing was 15.5 mph av, and 2nd ride with my brother in law was 30 miles at av 16.5 mph.
Have to say - with all the mileage I've been doing on the Zesty (typically 22/23 mile rides in just under 2 hrs, with 1300 - 1500 ft climbing)I thought I'd be quicker on the road.
Reading the above, makes me feel a bit happier. Ultimately I have very limited time to ride my bike. All of the mtb riding I do is after the kids are in bed. Given a time scale I will always prefer to ride off road, but if I can fit some long road rides, that can only be good for my fitness. It is hard not to get too obsessed with average speeds, though.