Understanding what DNS-Prefetch is: Developer.Mozilla.org
To make it happen, add this in the <head>
<link rel="dns-prefetch" href="https://www.example.com">
Test it with: WebPageTest.org
Results of DNS-Prefetch
The blue part of the load time is the DNS resolution time.
To see the real effect, test it from a
rare different location. Here is a test from Nebraska. As you can see in the screenshot below, the DNS resolution took quite a while and starting early, thanks to DNS-Prefetch saved precious load time.