But how do you find a courier? I don't live anywhere near where I'd be mailing my application. Why would it take up to 20 days if I just overnighted it?
There used to be lists of couriers registered with the consulate on the embassy (britainusa) web site, but people have reported that those lists are now not there anymore for some reason. There's been a lot of discussion here about couriers lately, though, so I'm sure you could find some names for your consulate simply by searching this board.
If you sent it in yourself, your app gets put into a queue- the consulates are very busy right now so your application probably won't even get looked at for a little while after it is received because they're looking at the other applications that came in before yours. If you use a courier, you effectively move your application up to the head of the line which is why it will only take a couple of days max if your app is straightforward.