I don't claim to be a fast or strong rider but I seem consistently ~1 mph slower on a Brompton M6R w/Marathons and half clips vs my Specialized Tricross Comp w/700x32 and clipless which is ~7%. I just finished rigging a lower more aero position for my Brompton's M bars that should reduce the gap significantly (an initial run proved promising).
I've used a Garmin 60csx on fixed loops through mildly hilly terrain several times to compare.