It can take forever, depending how many packages have changed since release, and mirror speed. Also pretty sure there's no checkbox, though that could have changed at some point.
(Fwiw, I'm not here to say "Windows good" or anything.)
Debian and Ubuntu installers do the same thing, and have for quite some time. You can cancel when you see it start grabbing apt sources, or just be offline.
Keep in mind that they can be pressured by others into doing something they don't necessarily "want" to do. Making your intentions clear can be useful.
A central principal of the fediverse is decentralization, and to that end, it's healthier to spread the users across many instances than to have folks concentrated too heavily on any one.
It's certainly healthier for the whole to have users spread across instances, but that's a matter of emergence. What's your pitch to convince an individual to choose a smaller instance, when it's far more likely that their instance will cease to exist, taking their identity, history and hosted communities with it?
Y'all know about libel?