Alexi Grewal, 1984 Olympic road race gold medalist, mentioned that a small wheel is more efficient than a large wheel in speeds up to 16 mph. In speeds between 16 and 33 mph, small and large wheels are equally efficient. Only in speeds faster than 33 mph is a large wheel preferable. They also accelerate like a rocket. When riding with my wife she always takes a mini ( super small wheel 16" folding bicycle ) and she can keep up with me. Climbing hills is much easier than on a big wheeled bike. I always fly by people on hills when riding my small wheeled folding bicycles.
The current one hour human powered vehicle ( bicycle ) record uses small 20" wheels. So if you want to go fast get a small wheeled folding bicycle!