Are folding bikes less efficient than full sized bikes? I've recently been touring with my Birdy folder, and people constantly ask me if it's hard work to tour on a folding bike. Compared to my normal sized bike (which has the same gearing, 8-speed internal hubs on both) I don't notice a big difference.

Given the same transmission, bike weight and riding position between a full size and folding bike, are folders less efficient?
The factors that affect bike efficiency are: 


Weight
Mechanical power train Losses
Aerodynamic Drag
Rolling resistance


For a touring bike, the difference between a folding and regular bike are all lost in the noise. Smaller wheels tend to have higher rolling resistance and the bike might be a bit heavier, but for touring it just doesn't matter that much.

I think what is happening is that people remember the terrible heavy small wheeled bikes they had as kids. ( Why we give 50lb people 30lb bikes is a mystery..) They associate the relative amount of work those bikes required with your "small" wheeled bike. And yes, if a folding bike was 50% or more of your body weight it would be a lot harder to tour with.