Ok,
so, I am convinced mudguards are a good thing, but I remain to be convinced that I need anything more (or anything less) than 2 gears...
I am inclined to go S2L
Have you had a test ride on one? I wasn't convinced about the 2-speed option until I actually went for a test ride...
I came *that* close to placing my order as soon as I got back to the shop, but luckily held off for a few days, and in the meantime my circumstances changed which meant I wasn't commuting regularly any more so couldn't justify the purchase... now I'm back to regular commuting, I'm seriously thinking I need to get one.
As for whether or not they're slow bikes, well, I won't be using one for time-trialling any time soon but my experience over years of regular commuting is that I was doing a lot of overtaking of other riders (more than I was being overtaken by). Perhaps the real answer is that there's no such thing as a slow bike, only a slow rider - after all, I even overtake a number of other riders on the Santander bikes and they handle like oil tankers, and have block and tackle gearing.
Currently, I'm riding my clunky old 7spd hybrid to the station at the home end of the commute, and I don't think that's any faster than the Brompton. The MTB is definitely slower than the Brompton.
I did a test of 'fast folders' last year and to be fair, the Brompton (S2L Superlight) was probably the slowest of the four I tried, but not by much. The Airnimal Joey (in drop-bar configuration) was easily the fastest - handles almost like a normal road bike - but it looks weird and folding it is a pretty convoluted process. The Tern Verge X18 (also with drops) is genuinely quick too but really unwieldy when folded. So the Brompton definitely wins out over those two when quick, compact folding is a priority.
My favourite of the four I tried was the Hummingbird. Astonishingly light and really zippy and nimble. Only real downside is that it doesn't have proper mudguard fittings so you have to use clip-ons. When folded, it has a longer, narrower profile than the folded Brompton, so you could squeeze it between seats on trains. If money were no object, I would have no hesitation buying a Hummingbird with the e-assist option - even with the motor, it's sub-10kg.