I'll admit the Schwalbes aren't as easy to cycle as the Bromptons (Kevlar or non-Kevlar), but I also find having to stop in the rain, in the dark, to change a rear puncture (always fun on a Brompton!) to have a definite effect on my cycling speed as well! I just found them way too prone to punctures, but maybe I cycle on more glass and other debris strewn routes than you.
(I find the Armadillos on my road bike to be even more bomb proof than the Schwalbe Marathons, but as far as I know there aren't any Armadillos that'll fit a Brompton).