I posted a couple of years back on my experience of being overtaken by a roady who was coasting while I was pedalling quite hard on the Brompton. I watched him shrink into the middle distance apparently without any effort. And I've spend a fair amount of time tinkering with my hubs, maximizing efficiency etc.. With small wheels one can feel the bike slow over rough ground (or anything other than smooth ground) as force is redirected vertically and ones' hands get pummeled.