The first time a user visits your site, it’s likely that they won’t have a DNS mapping for your IP stored in their browser cache, and it’s possible their ISP doesn’t have a result cached either. For many of your visitors, the Domain Name System will have to retrieve and return the DNS record from the authoritative server for your domain.
That takes time, and since DNS is such a fundamental part of how the Internet works, we want to keep the amount of time it takes to a minimum. There’s no point having a well-optimized site on great hosting if it takes several seconds for your browser to find out where it should be sending requests. (more…)