Long shot: can you check if xdg-desktop-portal-gnome is installed? If yes please remove it and see if it helps.
No, it’s not installed.
Someone posted a possible workaround: 482082 – Plasma takes an extra 33 seconds to start after entering password and logging in when choosing the Breeze Splash Screen - KDE / Plasma 6
Check if this works for you.
Yes, works! Now the desktop starts in a few seconds! Great! I hope this will solved by default in a near future!
Thank’s!
The fix for me here, on my main PC. Haven’t checked others yet.
I also tried the Oxygen splash, I had a very long-lasting bit of nostalgia there. 
This at least might narrow it down to the splash mechanism.
I did see this happening on neon Testing, but it stopped happening fairly recently - within the past week? Maybe something just hasn’t made its way into release packaging?
We have a couple of bug reports and other reports of this, and are actively investigating it. I can’t personally reproduce it, but a few of my colleagues can, and they’re on the case.
Thank’s for your work!
You’re very welcome!
This seems to have been solved by the recent updates in KDE neon - at least for me the desktop both in the X11 session and the Wayland session loads/starts without any excessive delay now.
Thank you! 
See Bluetooth support improvements #1 (!600) · Merge requests · Network / KDE Connect · GitLab for the cause.
TL;DR bad interaction between KDE Connect’s new bluetooth backend and DBus if the system does not have bluetooth hardware
Not for me! I still need to use “None” as Splash Screen.
It’s not a problem for me, just for your (and developers) information.
Sorry, but I cannot understand the relation 
I’ve got a working bluetooth adapter. The problem seems really related with the splash screen theme.
Updated today. Still not fixed.
I’m affected and have Bluetooth hardware too. If your adapter presents as unavailable when it’s turned off, that can trigger the issue as well.
Hi! I have not any bluetooth adapters (built-in or external), but 25 sec delay issue triggered. Motherboard Asus ROG Strix X570-F Gaming.
Updated now, bug fixed. Thanks!
I’m on Arch with KDE 6 (fresh install, not an upgrade from 5.x) and I’m having this slow login issue as of today. Bluetooth is working fine.
Try disabling the splash screen in systemsettings.
The root cause should be fixed in 6.0.2. Please test again after it’s released.
Good to know, and thank you so much for responding as I wasn’t sure this was an active thread since someone did get their issue resolved. Here is the funny part, I hadn’t checked here and I was playing with settings restoring themes to default just to see if it was a theme issue and nothing changed even after reboots so I put things back the way I had them and … now the issue has stopped. lol. I feel gaslit. 
Solved for me with 6.0.2