I came across this Raleigh 20 page:
The author contends that folding bikes are more efficient than larger wheeled bikes. Smaller wheels are better for climbing up hills, better for accelerating, and better overall when doing speeds up to 16 mph. He didn't really cite any studies to back this up.
Wanted to know if this was your experience? I've commuted with my folder sometimes and I've felt slightly more exerted than I have with my large 700c bike. It could have been just the weather or conditions. Haven't really tested this theory out.