Web Performance 101: JS, CSS, HTTP, images & fonts


There’re a whole 5 different methods of preloading, and all of them are suitable for different goals. Here’re what these methods do, in brief:

<link rel="dns-prefetch"> instructs the browser to make a DNS request for a server’s IP address in advance. This is useful for CDNs, Google Fonts, and all other cases when you know you’ll need a resource in a short time, know the domain it’s hosted at, but don’t know its exact path. In this case, resolving the server’s IP address in advance would save you from 50 to 300 ms.

<link rel="preconnect"> instructs the browser to perform the connection to a server in advance. It’s useful in the same cases when dns-prefetch is useful, but sets up a full connection and saves more time. The drawback here is that opening a new connection is pretty resource-intensive, so you don’t want to overuse this optimization.

<link rel="prefetch"> preloads and caches a resource in background with a low priority. This is useful e.g. to preload a JS bundle for the next page of an app.

<link rel="preload"> preloads a resource in background with a high priority. This is useful to preload a resource you’ll need in several seconds – e.g., a non-critical CSS file.

<link rel="prerender"> preloads the specified page in the background and renders it in an invisible tab. Later, when a visitor clicks to a link leading to the prerendered page, the page displays immediately. This is what Google uses to preload its first search result.

Note: don’t overuse prefetching methods. Downloading stuff in the background still consumes visitor’s traffic – and it’s really bad on mobile. So adding 10 extra preloads might make your app a bit faster, but your visitor will pay real money for this. 2-4 preloads per page are probably fine.

Read more: Preload, prefetch and priorities in Chrome · Prefetching, preloading, prebrowsing