My personal experience:
Last year, on a hybrid outfitted with road tires, I was averaging about 12.5 mph riding the same 6 mile loop around my neighborhood. I bought a road bike (Salsa Casseroll) and the first week that I had it I averaged 13.27 riding the same loop. I know the numbers are accurate because I was keeping track on 'Map my ride'.
I have to admit that I had similar tires on the hybrid and the road bike (700 x 32's), so the increase in speed was less drastic than if I'd had narrower road tires. I've switched to 28's on the Casseroll, but I can't compare speeds because I've moved and no longer ride the same loop.
As an aside: quit smoking when you want to, not because somebody else tells you to.