slow shutdown on Ubuntu 16.04, don't how to diagnose

Do you use a printer?

Yes I am. It's a printer on the network.

https://askubuntu.com/questions/760952/slow-shutdown-on-ubuntu-16-04-lts-stopping-thermal-daemon-running-fit-make-remo

Change the timeout as stated

Actually I already tried the methods metioned in the website you shared. Changing the timeout only make it worse. So I have to restore my system back to the point before I changed the timeout.

Is there any way to see which process is hanging during shutdown?

I think cups service is already disabled

jc@jc-XPS-15:~$ sudo systemctl status cups-browsed
● cups-browsed.service - Make remote CUPS printers available locally
   Loaded: loaded (/lib/systemd/system/cups-browsed.service; disabled; vendor pr
   Active: inactive (dead)

Okay I found some clue in my /var/log/syslog

This is the log when system hang during reboot. From 23:40:58 to 23:42:36

Jan 30 23:40:58 jc-XPS-15 systemd[1]: Stopping User Manager for UID 108...
Jan 30 23:40:58 jc-XPS-15 systemd[1]: Stopping Authenticate and Authorize Users to Run Privileged Tasks...
Jan 30 23:40:58 jc-XPS-15 systemd[1]: Stopping ACPI event daemon...
Jan 30 23:40:58 jc-XPS-15 systemd[1]: Stopping Save/Restore Sound Card State...
Jan 30 23:40:58 jc-XPS-15 systemd[1178]: Stopped target Default.
Jan 30 23:40:58 jc-XPS-15 systemd[1178]: Reached target Shutdown.
Jan 30 23:40:58 jc-XPS-15 systemd[1]: Stopping Manage, Install and Generate Color Profiles...
Jan 30 23:40:58 jc-XPS-15 systemd[1]: Stopped target Timers.
Jan 30 23:40:58 jc-XPS-15 systemd[1]: Closed Load/Save RF Kill Switch Status /dev/rfkill Watch.
Jan 30 23:40:58 jc-XPS-15 systemd[1178]: Starting Exit the Session...
Jan 30 23:40:58 jc-XPS-15 systemd[1178]: Stopped target Basic System.
Jan 30 23:40:58 jc-XPS-15 systemd[1178]: Stopped target Paths.
Jan 30 23:40:58 jc-XPS-15 systemd[1178]: Stopped target Timers.
Jan 30 23:40:58 jc-XPS-15 rsyslogd: [origin software="rsyslogd" swVersion="8.16.0" x-pid="831" x-info="http://www.rsyslog.com"] exiting on signal 15.
Jan 30 23:42:36 jc-XPS-15 rsyslogd: [origin software="rsyslogd" swVersion="8.16.0" x-pid="845" x-info="http://www.rsyslog.com"] start
Jan 30 23:42:36 jc-XPS-15 systemd-modules-load[363]: Inserted module 'lp'
Jan 30 23:42:36 jc-XPS-15 systemd-modules-load[363]: Inserted module 'ppdev'
Jan 30 23:42:36 jc-XPS-15 systemd-modules-load[363]: Inserted module 'parport_pc'
Jan 30 23:42:36 jc-XPS-15 systemd[1]: Starting Flush Journal to Persistent Storage...
Jan 30 23:42:36 jc-XPS-15 rsyslogd-2222: command 'KLogPermitNonKernelFacility' is currently not permitted - did you already set it via a RainerScript command (v6+ config)? [v8.16.0 try http://www.rsyslog.com/e/2222 ]
Jan 30 23:42:36 jc-XPS-15 systemd[1]: Starting udev Kernel Device Manager...
Jan 30 23:42:36 jc-XPS-15 rsyslogd: rsyslogd's groupid changed to 108
Jan 30 23:42:36 jc-XPS-15 rsyslogd: rsyslogd's userid changed to 104
Jan 30 23:42:36 jc-XPS-15 loadkeys[358]: Loading /etc/console-setup/cached.kmap.gz
Jan 30 23:42:36 jc-XPS-15 systemd[1]: Started Set console keymap.
Jan 30 23:42:36 jc-XPS-15 systemd[1]: Reached target Local File Systems (Pre).
Jan 30 23:42:36 jc-XPS-15 systemd[1]: Started Flush Journal to Persistent Storage.

Seems to be syslog slowing things down.