There seems to be a recent change in the function that removes obsolete kernels
when booting after a kernel update:
In the past the kernels were removed while the user was working.
Recently it seems the login manager waits until obsolete kernels are removed,
causing a significant delay.

Here is an example:
pc:~ # journalctl -b |egrep 'obsolete|logind'
Jul 11 08:38:02 localhost systemd-logind[2301]: New seat seat0.
Jul 11 08:38:02 localhost systemd-logind[2301]: Watching system buttons on
/dev/input/event7 (Power Button)
Jul 11 08:38:02 localhost systemd-logind[2301]: Watching system buttons on
/dev/input/event4 (Power Button)
Jul 11 08:38:02 localhost systemd-logind[2301]: Watching system buttons on
/dev/input/event5 (Lid Switch)
Jul 11 08:38:02 localhost systemd-logind[2301]: Watching system buttons on
/dev/input/event6 (Sleep Button)
Jul 11 08:38:02 localhost systemd-logind[2301]: Watching system buttons on
/dev/input/event0 (AT Translated Set 2 keyboard)
Jul 11 08:38:02 localhost systemd-logind[2301]: Watching system buttons on
/dev/input/event8 (ThinkPad Extra Buttons)
Jul 11 08:38:03 localhost zypper[2247]: Preparing to purge obsolete kernels...
Jul 11 08:39:12 pc systemd-logind[2301]: New session c1 of user gdm.
Jul 11 08:39:12 pc /usr/lib/gdm/gdm-x-session[17172]: (II) systemd-logind: took
control of session /org/freedesktop/login1/session/c1
...