I've today done a regular route I do approx weekly, instead of a roadbike but on a Brompton S6L-X. This is a S2L-X from 2006 with the transmission from a 2011 S6L.

route - http://ridewithgps.com/routes/811734

Videos and photos here

At my current level of fitness on my road bike I'd expect to do it in about 6hr30mins.
Today on a S6L-X Brompton with new transmission (BWR <500 miles, with its new chain and sprockets, new Marathon Plus tires, 44T Brompton crankset and SON hub), I took an hour longer (16% slower).

Some of that I can account for:
- wider gear jumps so either too-high or too-low so took too-low, slowing my uphills. If this were a shorter trip, much more within my fitness, I'd more inclined to take too-high.
- upright position on downhills, no drop-handlebars, slowing my flats and downhills.
- top gear of 88" , slowing my downhills, my roadbike has top of 108"
- Marathon Plus tires, slowing me the whole time.
- small effect is I had an S-bag, from experience it adds so little drag its nearly a fairing.

On the 1st/2nd gear (hub low) there is a grinding noise, indicates some losses. In all other gears there is some kind of clicking. On downhills, the hub is making the chain hang so I've been putting in middle hub gear when freewheeling hoping it reduces drag?

The good news is a stock Brompton 6speed can at least *DO* such terrain, 66 miles, 7000ft of climbing, average speed including stopping to eat/drink, talk, and take photos, is 8.8mph vs a roadbike 10mph. If I assume 30mins spent stationary, then average speed 9.5mph vs a roadbike 11mph, which is largely in the my ballpark experience the Brompton is about 2mph slower.

The BWR according to reports in forums will improve, and I know a new set of Marathon Plus will improve as the terrible tread, Schwalbe feel we need, wears down to get nearer to slicks.

I've also built a S6R with SRAM and Mountain Drive, it has closer gears and a higher 97" top gear, it should be faster? However, when I was fitting the MD to the frame, when no chain hooked on, I could perceive resistance to rotation, MORE than when having a Brompton crankset on a *** BB (I typed the letters F A G which is the name of the manufacturer but this forum replaced with *!) , so the Schlumpf *IS* producing some losses even in its direct-drive mode, that could well cancel out many of the performance gains it offers (higher top, closer gears with SRAM). I might have a go at applying some grease as per service instructions to the Schlumpf although it should NOT need it as its done very little mileage since new from the factory.