I went on a quest similar to yours after I discovered folders about 7 years ago. Ended up going through about 25 folders looking for something that could simulate my 17lb carbon road bike. The closest that I found was the Bike Friday Pocket Rocket and the Moulton TSR. I also used a Dahon SmoothHound as a roadie substitute for a long time, but it's not a folding bike. Each of these bikes are still heavier than my fast road bike.

The small wheels spin up faster, but don't hold speed as well. I think the small wheels are advantageous for climbing. I also think drafting is better with small wheels as you can get closer to the bike in front. I concluded that folding bikes will never replace a high-end road bike, but they certainly will give you a great ride and workout if you need the convenience of folding for transportablity or storage.

I've switched to recumbent riding now and have sold off all my upright bikes except for the Mouton TSR (with Capreo hub) and a Brompton. That Moulton space frame is superior in ride quality, comfort and stiffness. For a century ride, it's the perfect (non-recumbent) bike. They're beautiful, too