To disable it, you will need to edit the following file: /etc/modprobe.d/aliases
sudo vim /etc/modprobe.d/aliases
And find the 'alias net-pf-10 ipv6' line, and repleace it with the following:
alias net-pf-10 off ipv6
Next step is to reboot, and see if your internet connection id any faster.
To check to see if IPv6 is running, run the following command:
ip a | grep inet6