how much faster on a road bike?

Depends.
But FWIW I've done a 10 mile time trial on my mtb (very racey but still nobbly tyres at 50psi) in about 26 minutes. I could do it on my normal road bike in about 22. I'd expect that on proper nobblies at proper pressures it'd be more in the 29-30 minute range.

Quite a bit quicker
 
I ride to work regularly. 15km one direction, 100% tarmac, 100hm hill at the end. I have an average speed of 20-22 km/h (measured for a few months), depending on the suspension. More suspension = slower.
On my road bike I always end up around 27 km/h. But I always had the impression that I was slightly more exhausted when coming home. It seems I can not relax when riding the road bike. So that might be part of the explanation.
 
Re:

The difference in average speed over same course is less than youd think, about 3 to 5 mph.
 
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