I see the same on 15.2 and Tumbleweed.

On login the splash starts, then the login sound plays then apps are started and display if I logged out on the virtual desktop that holds them but no wallpaper or taskbar. I can use any apps that are visible, switch between windows but if I minimise they vanish.

Eventually the wallpaper/desktop displays as does the taskbar and it works fine going on. If I logout and back in it can startup quickly as expected.

I start kmail, konversation, kleopatra and amarok/clementine at login (or rather they get started as shutdown with them running.

I point wallpaper slideshow to my whole photo selection which is pretty big, maybe it's that. They are on NTFS partition shared with Windows. (Would be nicer if Windows could work with Linux partitions.)