Slow shutdown due to modemmanager and .. nm-dispatcher.action?
Here is the bug report on, no sign of a fix.
https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/869635
The report does mention an edit one can make to /etc/init/network-manager.conf, ie. add as shown
stop on runlevel [06]
I believe adding the line was effective here though myself use Wicd & have removed network-manager. YMMV, if trying & it causes issues then remove the added line