Preload key requests

The Opportunities section of your Lighthouse report flags the third level of requests in your critical request chain as preload candidates:

Preload key requests
Preload key requests.

How Lighthouse flags determines preload candidates

Suppose your page's critical request chain looks like this:

index.html
|--app.js
|--styles.css
|--ui.js

Your index.html file declares <script src="app.js">. When app.js runs , it calls fetch() in order to download styles.css and ui.js. The page doesn't appear complete until those last 2 resources are downloaded, parsed, and executed. Using the example above, Lighthouse would flag styles.css and ui.js as candidates.

The potential savings are based on how much earlier the browser would be able to start the requests if you declared preload links. For example, if app.js takes 200ms to download, parse, and execute, the potential savings for each resource is 200ms since app.js is no longer a bottleneck for each of the requests.

Preloading requests can make your pages load faster.

Without preload links, styles.css and ui.js are requested only after
            app.js has been downloaded, parsed, and executed.
Without preload links, styles.css and ui.js are requested only after app.js has been downloaded, parsed, and executed.

The problem here is that the browser only becomes aware of those last 2 resources after it downloads, parses, and executes app.js. But you know that those resources are important and should be downloaded as soon as possible.

Declare preload links in your HTML to instruct the browser to download key resources as soon as possible.

<head>
...
<link rel="preload" href="styles.css" as="style">
<link rel="preload" href="ui.js" as="script">
...
</head>
With preload links, styles.css and ui.js are requested at the same time
            as app.js.
With preload links, styles.css and ui.js are requested at the same time as app.js.

See Can I Use link-rel-preload? to see browser support for preload links.

See also Preload critical assets to improve loading speed

More information

Last updated: Improve article