I've recently updated my Manjaro system and I've noticed an unusually slow shutdown process, around 2 minutes.

I've found out that it is an issue of systemd update. To workaround it just add this:

Slice=-.slice

To the end of this file:

sudo nano /usr/lib/systemd/user/gnome-session-restart-dbus.service

Credit: https://forum.manjaro.org/t/pretty-slow-shutdown/41785/2