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!