Below is a quote from captainsticks which has links which describe how to change it.
Remember that during the installation of CIS you will be asked if you want to install Secure DNS, it will be ticked in by default so you have to un-tick it.
Edit: Though since you uninstalled CIS without changing the DNS settings and the site started working normally, I am wondering if it’s really the DNS which brings the problem though I wonder what else in CIS could slow a certain website down :-\