Usually you load some stuff that comes outside your domain then website needs to resolve that domain and it takes few seconds or even minutes.

Using DNS Prefetch you can do that faster by telling to the browser handles some domains and resolves them smarter.

Examples:

<link rel="dns-prefetch" href="//www.google-analytics.com">
  • Here you can see a good article speaking about dns-prefetch and few more tips to speed up its website loading.