It can be reproduced with. dbus-send --print-reply --dest=org.kde.klauncher5 /KLauncher org.kde.KLauncher.start_service_by_desktop_path string:"/opt/kde5/share/applications/kinfocenter.desktop" array:string: array:string: string:"" boolean:false (the call we make from dbus) we should see a response immediately or at least when kinfocenter loads. Instead everything blocks until dbus timeout occurs.
It turns out the application was at fault. The .desktop file was marked as DBus unique, but it was ported away from KApplication which registered the dbus path. kdeinit was waiting for it to register the dbus name.