It really depends in the bike and fitness. Conversion of a knobby-tired MTB to slicks seems to result in a roughly 20% increase in speed (12 to 14mph OR 14 to 17mph moving average.) Gearing and wheel size is a also a huge difference. I got a slight bit of time on a German trekking bike this weekend (think wide 700x40 tires and 48T up front) and it makes a world of difference.

I was able to click off 30km much easier than 30km on my MTB. (I think the Moving average was 28km/h or so, compared to 22km/h, with much less effort).