I used to average 20mph in London on my road bike, which had 19mm tyres. You'll also find that few MTBs can keep up with one on road because of higher rolling resistance, more weight and less efficient position (unless the MTB rider is very good!), also, road bikes tend to have bigger chainrings/gear ratios (my road bike has a 54 front chainring, which is a bit painful now I'm in the hills!). As for potholes, it isn't a good idea to hit them with such skinny wheels - I used to jump them.

However, unless you've got the space for a 2nd hand racer, just get some semi slicktyres (make sure there's some rain-treads!) and you'll speed up by 1-2mph!