Heavy load by APT unattended-upgrade

The functionality on unattended-upgrades is automatically enabled after the installation of 16.04 Xenial Server. I mistakenly didn't really disable it so observed the unexpected behaviors above. Method Two here I had followed is unlikely effective.

To properly disable the automatic updates, we need change the configuation on /etc/apt/apt.conf.d/20auto-upgrades from:

APT::Periodic::Unattended-Upgrade "1";

to:

APT::Periodic::Unattended-Upgrade "0"; 

If your OS uses systemd then:

sudo systemctl mask unattended-upgrade.service

would also stop it from automatically running.

The main issue I have is: why is it taking up 100% CPU for downloading some files?