I suppose it's some kind of carryover from drupal 7, because there it worked exactly the same on one of my sites (dev), while it was ok in production.
Maybe someone has idea why adding new field to form from backend takes between 15 and 20 minutes to render the page with field settings? I'm not even sure how to debug this, last time when I enabled all queries in devel module I think it were cache queries which took the longest. But nowhere near 20 minutes...
Are you using the profile contrib module? I haven't tried it. In core you can already add fields to a user account. Have you compared the two?
I can add fields, but I will need probably around ten profiles, each one with separate registration page, so core is not an option.
I thought that profile module was part of the core now, but I must have mistaken drupal 8 with backdrop, then.
I think profile is still in contrib for Drupal 8 https://www.drupal.org/project/profile.
I've tested the Backdrop profile contrib module and didn't notice any slowdown. I can't recreate your issue. I suggest comparing with a fresh install that has just profile module. Something else is going on I suspect.
Thank You for trying :)
Recreating will be the hardest part, because bug may be well hidden, or it may come from unfortunate clash of some modules.
Never mind, I'll just exercise my patience.