I did the same route today, but instead of a S6L-X/BWR-44T/SON, I did it on a S6R/SRAM/SchlumpfMD-56T. The elapsed time was higher. So even so closer gears to be nearer to ideal cadence, and a higher gear to help on downhills, the fact of adding a hub (Schlumpf Mountain Drive) was taking more away overall? Still it felt easier on the steepest climb nearer the end, as I had lower gears when I wanted, and if I were lugging camping gear, I'd need those gears. For all but a few short steep sections, I never went below my 3rd gear.

Photos and videos

So my conclusions overall:

1. Don't use a folder like a Brompton unless folding is a required/possible part of the route (I knew that already, just good to know the relative effect)
2. fewer and simpler hubs to get just enough gearing is better
3. add more gearing if the terrain or weight of kit will require

The only surprising bit is I thought I'd be same/better with SRAM+MD than BWR, I'd heard that BWR was less efficient and that Schlumpf efficient but seems just adding more hubs is more drag, so only have them fitted if you NEED them.

I'm wondering if I should build a hub-less Brompton, I can actually make a 2-speed rear with a front-derailleur 30T/54T (I have the parts..... .)

Seriously, I was simply using the Christmas holidays to try out combinations, I'm going to use a roadbike for when folding is not a part of the day and at least I know the relative speed impact if I want some folding flexibility, its also good to know the bikes I built the last week can actually handle the gradients and distances. Slower, but no less capable. The Touring Brompton I built is going to be used probably less than 10 days this year. The day-bike is going to be go-to bike if folding is needing, which it tends to be on commuting and shopping trips, and my M2L-X is probably going to be the children's folder as they are less muscly for lugging the 1Kg more of a 6-speed.