$ cat /etc/lsb-release DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=16.04
DISTRIB_CODENAME=xenial
DISTRIB_DESCRIPTION="Ubuntu 16.04.2 LTS"
を利用しているんですが、shutdown起動から電源断までの時間が
遅くて困っています（というか電源が切れない）

前回終了時のログは以下の通り
/var/log$ tail ./syslog
Jul 30 15:10:27 mobile systemd[1]: Starting Stop ureadahead data collection...
Jul 30 15:10:27 mobile systemd[1]: Started Stop ureadahead data collection.
Jul 30 15:10:27 mobile systemd[1]: Stopped Read required files in advance.
Jul 30 15:10:33 mobile systemd[1393]: Time has been changed
Jul 30 15:10:33 mobile systemd[1327]: Time has been changed
Jul 30 15:10:33 mobile systemd[1]: Time has been changed
Jul 30 15:10:33 mobile systemd[1]: snapd.refresh.timer: Adding 5h 11min 40.570927s random time.
Jul 30 15:10:33 mobile systemd[1]: snapd.refresh.timer: Adding 2h 12min 42.090233s random time.
Jul 30 15:10:33 mobile systemd[1]: apt-daily.timer: Adding 5h 55min 46.866304s random time.
Jul 30 15:10:33 mobile systemd-timesyncd[739]: Synchronized to time server 91.189.89.199:123 (ntp.ubuntu.com).

設定周りで見なおしたのは以下の部分です
$ grep DefaultTimeoutStopSec /etc/systemd/system.conf DefaultTimeoutStopSec=10s

あとどこを確認すべきでしようか、アドバイスお願い致します