I've spent a long time with folding bikes over the years. My advice: bite the bullet and buy a Brompton[1]. Yes, Bromptons are expensive. But they ride like a bicycle, have gears, a stiff enough frame, luggage racks etc. There's very little compromise. The fastest I've done on mine is 45 kph and I've ridden 70km in a day. I carry it on and off buses and trains multiple times a day without issue. In the past I had an…
The fold on a Brompton is just about perfect. It takes no time at all. I'm a little annoyed to find that they're using copyright to keep a monopoly on it, but certainly not so much that I'm going to give it up.
I tried out a Tern, a Dahon, and some random ancient thing. I've never regretted spending the extra coin on the Brompton. The fact that all the greasy bits are in the middle when it's folded means you throw it in your trunk without thinking about it. It means you put it between your legs on the subway without thinking about it. The fact that it folds in seconds means you don't miss your train fumbling with it at the fare gate.
The only downside I can see is like half the parts on the thing are bespoke. Replacing a derailleur, for instance is not something your average shop can do. Ditto the shifters.
It's the Apple of bikes: the tight integration of the components gets you an absolutely magnificent package (at a premium price), but you get some lock-in as a result.