It seems ipv6 is enabled by default in ubuntu and I've also heard that people are complaining that it slows them down. Here is a quick way to get rid of it.

vim /etc/modprobe.d/aliases

Look for the line which says alias net-pf-10 ipv6 and change it to alias net-pf-10 off ipv6


Restart your network or reboot for the change to take effect