At times, I'm able to pass roadies that are going slow. I can usually hang with or pass someone riding a mountain bike on the street. No doubt about it, though, on my DT, I'm probably 20% slower than on a road bike. However, on my new (used) Air Friday, I'm very close to the same speed as my road bike. Tires are huge, gearing is huge, and your position on the bike is huge. Put them all together and they really add up.
FWIW, On my Downtube, on flat pavement with no wind, I run 53/13 or 53/12 gearing. I have to have wind/downhill to get me into the 53/11. On the Friday, I can run in my 60/13 or 60/15 gear on my own. Pretty big difference.