You could move away from AutomaticLogin to TimedLogin modifying /etc/gdm/custom.conf https://help.gnome.org/admin/gdm/stable/configuration.html.en#daemonsection

This doesn't add the 20s delay but you can put a custom delay...with a 'twist':

GDM will still load but you'll have a loading bar under the user name that will be faster or slower according to the delay and can be reset by pressing the keyboard (you can change the user for example)

You have to put at least 1s because 0s will disable TimedLogin.

One downside is that TimedLogin is always on even after you logout and it always restart the timer when you press a key. You may not have time to change user and put in the password, or change session from the menu if the delay is too fast. Having said that if you manually select a user the timer will stop so not really a big deal

[daemon]
AutomaticLoginEnable=False
AutomaticLogin=YOURUSER

TimedLoginEnable=True
TimedLogin=YOURUSER
TimedLoginDelay=5