I am also new to this road bike game and was commuting 26 miles a day 2-3 times a week on a hardtail with semi slicks inflated to 60psi. I have now bought a road bike for the commute in fair weather and use the mtb if wet.
I have found that my average speed is 3mph more on the road bike - approx 2+ miles is through town and both bikes are the same
! This is an improvement of 15%. The road bike is massively easier to pedal, particularly into a head wind and like
joolsybools my legs don't ache at the end of it
. The other major difference is the acceleration on the road bike - just awseome compared to the hardtail. Having said all that, I was a bit dissapointed the difference wasn't greater but I think my speed on the road bike will improve and the difference between the 2 may well widen.
Finally, the road bike is a shear joy to take out for spin - a commute has to be done, but I now WANT to get out on the road, just for fun!
My advice would be to start looking a road bikes, try lots and make use of the cyclescheme! You won't regret it