Occasionally when restarting or shutting down (about 1 out of 5 times) the PC takes about 1-2 minutes to restart instead of the usual 2-3 seconds.
This screen appears and keeps loading something until it shuts down after a minute or two.
I have no idea what is causing this, initially I thought maybe it's updates that are being finalised. But It happens even when I haven't installed anything and it's hard to pinpoint exactly when it happens.
In my experience it is because there are applications that have open files. A sure-fire way to test this is for you to close all applications before shutting down and restarting.
Some other people have found this to be caused by the CUPS software (related to remote printers) here: Slow shutdown on Ubuntu 16.04 LTS (Stopping thermal daemon/running fit make remote CUPS printers)
If that's what the problem is for you too, the short answer is:
sudo systemctl disable cups-browsed.service
