Jipe

From my experience of riding small wheel bikes, Brompton, Birdy, Moulton, Tern GSD and also a Hase Pino semi-recumbent tandem with a 55x406 front wheel, the only situation in which a small wheel is a major problem is riding off road on soft ground, like sand and mud where at a certain point small wheel cannot ride anymore. The Hase Pino which is heavily loaded with the two cyclist on it is the most sensitive to this.

On rocky trail with big stones and potholes, its also a problem, but none of my bikes is made for such situation.

For the speed comparison, the Brompton is a bad example, its significantly slower and less efficient than a Birdy or Moulton and several models of Bike Friday.

prozacist

20" folder, in my experience, is about 30% slower than 700c.

Jipe

This question is for me meaningless because not all 700c bike have the same performances, some are faster than others, and its the same for 20" bikes!

A comparison only mean something if the types of both bikes is mentioned (including wheel-set and, the most important, the tires types mounted on both bikes for the comparison) and the performances difference is only valid for the mentioned set of bikes with the components used for the comparison, just changing the tires will change the comparative performances.