There has been some weird delay when logging in/out. To find out the cause, I have disabled everything such as Splash, background services, autostart apps, etc. I even removed all files, widgets, and wallpaper from the desktop. It still happened. When I tried to log in, there is about 7 second black screen, when logging out, the system does nothing for about 10 seconds (mouse is moving).

After a lot of trial, I finally found a clue. When I change the desktop's Layout from Folder View to Desktop, then this did not happen. Now, how to fix this? Strangely, when I created a new user account, it did not happen even when the default settings was Folder View.

Operating System: Manjaro Linux
KDE Plasma Version: 5.14.3
Qt Version: 5.11.2
KDE Frameworks Version: 5.52.0
Kernel Version: 4.19.4-1-MANJARO
OS Type: 64-bit
Processors: 4 × Intel® Core™ i5-3570K CPU @ 3.40GHz
Memory: 15.6 GiB of RAM

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I found out the reason. For some reason (probably some bug), the ~/.config/plasma-org.kde.plasma.desktop-appletsrc got big (1.8MB) and contained a weirdly long line. So, even opening the file in Kate made Kate hang. I deleted that file, and now the weird delay is gone. I hope this would help others who would encounter the same problem in the future.