More takes on using a "folding" bike for touring.
About 12 years ago I bought a Moulton TSR frameset, that I built up with the same type of gearing as Mickeg, a SRAM dual-drive with 3 internal gears and a 7 or 8 speed dérailleur cassette. The TSR uses 20" wheels, it performed well on fast day rides and was quite good for on-road touring with a reasonable amount of luggage fitting onto the Moulton specific front and rear racks. But it wasn't as good as a fat-tyre full size bike for rocky or sandy paths and had less total luggage capacity, so a bit constrained for camping or for holidays with my wife, when I carry most of the luggage.
Rather than fold, the frame of the Moulton TSR could be split in the middle, and the seatpost and bars removed to make a package a bit smaller than a full-size bike with the wheels removed. I found dismantling the bike inconvenient in practice and hardly ever used this feature.
I already had Bromptons for commuting and survey work. The fold is very compact and quick, but a Brompton is less comfortable to ride than the full-suspension Moulton TSR and a full size bike with 26"x2" tyres. The Brompton gear range was also insufficient for some types of touring.
So I rationalised my bike stable, and sold the Moulton on (they keep their value well). I bought a Thorn Raven Tour with 26"x2" tyres, while tweaking one of my Bromptons to make it slightly more comfortable and more suitable for hilly touring.
The current iteration of my touring Brompton has titanium forks and handlebars to improve comfort and a custom rear triangle with integrated rear rack and Rohloff hub. It is as quick to fold as a standard Brompton and only slightly wider, so still very compact. Weight is about 12.3 Kg without luggage. Compared to my Raven Sport Tour day bike, there is a slight performance penalty. I haven't got reliable figures for this, but I reckon about 5% slower with a similar light load, partly because of the more upright riding position on the Brompton, partly because of the less efficient tyres. Luggage capacity is quite good, about 28 litres in the front bag (which is mounted on a luggage block on the frame, so more stable than front panniers), when more capacity is needed I carry a 48 litre rucksack bungeed to the rear rack and stabilised at the top with a strap to the saddle.