I'd say look at your rpm (cadence) as many have suggested, around about the 90rpm is about right, but some people do prefer to be a bit lower. Another option (slightly strange approach) would be to take the slicks off and put some big MTB tyres on, that would increase the rolling resistance, slow you down a bit and therefore slow down your pedalling speed, good for your fitness, not so good for your speed.
The smallest gear you can get on a screw-in freewheel is 14. There's no room for anything smaller.
Is this a new thing for screw on freewheels or blocks as we used to call them, I had a 13 small sprocket on my old road bike that I raced on, 54 X 13 was my sprinting gear, I'm sure that you could get a 12 sprocket.
Giles