DNS stands for Domain Name Server. It is an internet equivalent of our phone book. It maintains a directory of Domain Names and translates them into Internet Protocol (IP) addresses. The Internet only recognises IP address and is dependent on the DNS to convert Domain Names into IP Addresses.
By default, the chosen DNS will be the Internet Service Provider (ISP)'s phonebook. Some experts claim that this phonebook is not powerful enough and needs slightly longer to translate Domain Names to IP Addresses. Hence slowing down the internet speed for the average user.
This is extremely crucial for me as I am on a 100 MB data plan with the "green pipe". Any small loss would mean a much slower surfing experience for me and my family.