I tried to dig into this annoying delay on any reboot by enabling persistent journald logs.
Here is what I found on shutdown / reboot path:
Aug 16 01:02:40 myhost org.kde.KSplash[2089]: org.kde.plasma.waitforname: WaitForName: Service was not registered within timeout
Aug 16 01:02:40 myhost dbus-daemon[2011]: [session uid=1000 pid=2011] Activated service 'org.kde.KSplash' failed: Process org.kde.KSplash exited with status 1
Aug 16 01:03:26 myhost systemd[1]: session-3.scope: Stopping timed out. Killing.
Not sure what that exactly is (login splash is disabled for me in system settings). I might report this as a KDE bug upstream.
Regards,
Shmerl.