Me and My Experiments..
On U16.04, I found that apt took really long time to update over IPv6.
Enter below command to force the use of IP v4 for apt
echo "Acquire::ForceIPv4 "true";" | sudo tee /etc/apt/apt.conf.d/99force-ipv4 > /dev/null
Thanks for posting comments.. - Raghu Nayak