I have 2 monitors. For both I set up wallpaper slideshow. One desktop has 9000 pictures, the other one 11000 pictures. When I start computer and log in, it takes about 3 minutes to show desktop and bottom panel. Computer is not frozen, because I see widgets, autostart applications are running and I can start other applications via krunner and use them normally.
STEPS TO REPRODUCE
1. Configure desktop - Folder View / Slideshow / Scaled and Cropped / Random
2. Add folders with 10000 images
3. Restart computers
It takes long time to start and see wallpaper and panel.
Wallpapers shouldn't stall plasma loading.
Linux/KDE Plasma: Neon 5.17
(available in About System)
KDE Plasma Version: 5.17.0
KDE Frameworks Version: 5.63.0
Qt Version: 5.13.1
Didn't have this problem with Plasma 5.16
I can also confim this.
As soon as you switch it to static color instead of random wallpapers the login process is much quicker and the panel, widgets, etc appear quicker.
There's also his link where people mention this problem : https://forum.kde.org/viewtopic.php?f=309&t=162793
*** This bug has been marked as a duplicate of bug 371455 ***