Trying to change login background in SDSM doesn't work. Clicking tiny button (wallpaper/background) in SDDM settings, makes CPU Fan go C R A Z Y and nothing happens. What is going on? Total broken and unusable. Why changing a simple walpaper/background would not work and eat CPU like a whale?
Have you read the Arch Linux wiki, specifically the section dedicated to SDDM theme (https://wiki.archlinux.org/title/SDDM#Customizing_a_theme)?
Personally, I've never had this problem...