Three weeks ago I switched from Debian Buster to the latest Stable called Bullseye.
On Buster I never had this issue. I suspected many things but after uinstalling and installing KDE-Connect multiple times using different Nvidia/Nouvea drivers I came to the conclusion it is most likely a KDE-Connect problem.
These are four scenarios:
1) Logging out from GUI is instant. If I restart/shutdown then it will most likely be instant as well.
2) Restarting from GUI takes 9 seconds with a black screen and moveable cursor.
3) Shut Down from GUI has the same outcome as Restart.
4) Restarting/Shutdown via the terminal is instant.
This issue appears on Debian's Stable, Testing, and Unstable versions.
Again, this issue was never around on Buster (old-stable)
Operating System: Debian GNU/Linux
KDE Plasma Version: 5.21.5
KDE Frameworks Version: 5.86.0
Qt Version: 5.15.2
Kernel Version: 5.14.0-1-amd64
OS Type: 64-bit
Graphics Platform: X11
*** This bug has been marked as a duplicate of bug 432643 ***