I turned on the pattern recognition part of my brain, played around a bit, and it seems like it hangs indefinitely at startup whenever --backends contains snap right after packagekit, otherwise it works:
plasma-discover --backends kns,packagekit,snap,flatpak,fwupd # Hangs. Btw this is the default on my machine.
plasma-discover --backends kns,packagekit,snap # Hangs
plasma-discover --backends packagekit,snap # Hangs
plasma-discover --backends snap,packagekit # Works
plasma-discover --backends packagekit,kns,snap # Works
plasma-discover --backends snap,fwupd,packagekit