If there is an person logged into the PC and I do:
Switch User > Start New Session
it takes 3-4 minutes to get a login prompt… (Fedora 31 and KDE)
I tryied and ended up forcing my machine to reboot. But it was with ~90% of RAM being used.
After reboot i can switch users in seconds.
What about RAM utilization?
I haven’t checked RAM. The screens go black but are still on. What I usually have to do is toggle between Ctrl + Alt + F1 and Ctrl + Alt + F4. This usually brings up the login window with the options to pick a login user but that disappears almost immediately. Then if I wait for another minute or so, hitting enter will bring up a login prompt.
I am thinking now that this is related to my user account, there must be something that the account is doing like mounting a non-existant share and then timing out… will investigate.
I had same behavior twice on F30. First time was when I had 50GB disk transfer between two different disks in same PC going on in the background. It all fixed itself as soon as transfer was done.
Second time was when I tried to overthink vm.dirty_ratio & vm.dirty_background_ratio to no longer have problems similar to one above 
In your case I’d check with iotop or something if there is no huge disk transfer going on in the background and if not then try to adjust vm settings.