I ride a 1990 era no shocks Diamondback 'ascent ex' mtb with 'Specialized Crossroads ex' 26"x1.95" tires at 80psi (flat center tread). It weighs 33lbs (!) as I have it set up. For my 9mi 2x per day commute I consistently travel at 20-22mph between stops, for an average speed (including the speed up and slow down time for stops) of ~18mph. I always use the 36 chainring with a 13-30 cog, usually on the 13-15 cogs, resulting in a 110 cadence.
This Thursday I will hopefully start using a Lemond Poprad set up with 700x28mm road tires. I will report back on how my cruising and average speed changes as I am very curious myself. A true lightweight roadbike (Poprad is a cyclocross bike) with even smaller tires would even be a slightly better of an improvement I imagine.
Al