It's a performance boost, albeit a small one. It (the browser) prefetches the domain so that when you request something from that domain (e.g. JS, CSS, fonts, images) it will already have the DNS information.

This doesn't have to be just for third-party scripts either, I also use it on subdomains of my website such as assets.myapp.com.

DNS prefetch is an attempt to resolve domain names before resources get requested.When a browser requests a resource from a server, that cross-origin’s domain name must be resolved to an IP address before the browser can issue the request.

For more information, refer the following link :

developer.mozilla.org/en-US/docs/Web/Performance/…