Can't give any percentages, but I got back into road biking via the MTB - here is what I noticed.
I started going some distance on the road with my MTB, but noticed that on knobbies, the bike would only go so fast, and then you seemed to hit a brick wall - it just wouldn't go any faster.
I put on a set of Bontrager slicks - fairly cheap and light. The bike (Trek Y22) definitely cruised at a higher speed, and rode a lot smoother. The only real problem I had was that after a few times on the slicks, I had an insatiable desire to get a serious road bike.
Also be aware that MTB riding doesn't necessarily give you good endurance. I thought I was in good shape, until some serious roadies led me on a 20 mile ride with lots of long hills. I've since acquired much better endurance, but that first long ride was a real eye opener.
One of my riding buddies still rides with slicks on his Cannondale hardtail, and does pretty good with it, so you don't absolutely have to get a road bike. Another, we can't seem to get out on the road, but the other two of us got road bikes, and never regretted it.
We live in and about Lexington, KY - so drop me a line if you want to join us sometime. Lots of terrific backroads around here. We still hit the singletrack in Frankfort on occasion, too.
--John