Using Lighthouse To Improve Page Load Performance

Lighthouse is an automated tool for improving the quality of your site. You give it a URL, and it provides a list of recommendations on how to improve page performance, make pages more accessible, adhere to best practices and more. You can run it from within Chrome DevTools, as a Chrome Extension, or even as a Node module, which is useful for continuous integration.

For a while now, Lighthouse has provided many tips for improving page load performance, such as enabling text compression or reducing render-blocking scripts. The Lighthouse team continues to ship new audits to give you even more useful advice for making your sites faster. This post is a roundup of newer performance audits that you may not be aware of, including:

• Main Thread Work Breakdown
• Preload Key Requests
• JavaScript Boot-up Time is High
• Avoids Page Redirects
• Unused JavaScript
• Uses Inefficient Cache Policy on Static Assets
• Avoid Costly Multiple Round-Trips to Any Origin
• Use Video Formats for Animated Content

Main Thread Work Breakdown

If you've ever used the performance panel in DevTools, you know it can be a bit of a chore to get a breakdown of where CPU time was spent loading a page. We're pleased to announce that this information is now readily and conveniently available via the new Main Thread Work Breakdown audit.

This new diagnostic evaluates how much and what kind of activity occurs during page load, which you can use to get a handle on loading performance issues related to layout, script eval, parsing, and other activity.

Preload Key Requests

When browsers retrieve resources, they do so as they find references to them within the document and its subresources. This can be suboptimal at times, because some critical resources are discovered rather late in the page load process. Thankfully, rel=preload gives developers the ability to hint to compliant browsers which resources should be fetched as soon as possible. The new Preload Key Requests audit lets developers know what resources could benefit from being loaded sooner by rel=preload.

It's super important you test and compare performance changes with and without rel=preload, as it can affect loading performance in ways you might not expect. For example, preloading a large image could delay initial render, but the tradeoff is that the preloaded image will appear sooner in the layout. Always make sure you're cool with the results!

JavaScript Boot-up Time is High

When too much JavaScript is loaded, the page can become unresponsive as the browser parses, compiles, and executes it. 3rd-party scripts and advertisements are a particular source of excessive script activity that can bog down even powerful devices. The new JavaScript Boot-up Time is High audit reveals how much CPU time each script on a page consumes, along with its URL:

When you run this audit, you can also enable third party badges in the network panel and filter the list to identify third party script resources. With the data from this audit, you'll be better equipped to find sources of excessive JavaScript activity that turn pages from snappy to laggy. For scripts specific to your application, you can employ techniques like code splitting and tree shaking to limit the amount of JavaScript on each page of your site.

Avoids Page Redirects

Sometimes when a browser requests a URL, the server can respond with a 300-level status code. This causes the browser to redirect to another URL. While redirects are necessary for SEO and convenience purposes, they add latency to requests. This is especially true if they redirect to other origins, which can incur additional DNS lookup and connection/TLS negotiation time.

Redirects are undesirable for landing pages on your site. To help you reduce latency and improve loading performance, Lighthouse now offers the Avoids Page Redirects audit, which lets you know when a navigation triggers any redirects.

Note that this audit is difficult to trigger in the DevTools version of Lighthouse, because it analyzes the current URL in the address bar of the page, which reflects the resolution of all redirects. You're likeliest to see this audit populated in the Node CLI.

Unused JavaScript

Dead code can be a serious problem in JavaScript-heavy applications. While it doesn't pose execution costs as it's never invoked, it does carry other undesirable effects. Dead code is still downloaded, parsed, and compiled by the browser. This affects loading performance and JavaScript boot-up time. Similar to the coverage panel in DevTools, the Unused JavaScript audit reveals JavaScript downloaded by the current page, but is never used.

With this audit, you can identify dead code in your applications and remove it to improve loading performance and reduce system resource usage. Pro tip: You can also use the code coverage panel in Chrome's dev tools to find this information!

Note: This audit is off by default! It can be enabled in the Node CLI by using the lighthouse:full configuration profile.

Uses Inefficient Cache Policy on Static Assets

While much performance advice tends to focus on boosting the speed of a website for first time users, it's also important to use caching to improve loading performance for returning users. The Uses Inefficient Cache Policy on Static Assets audit inspects caching headers for network resources, and notifies you if cache policies for static resources are substandard.

With the help of this audit, you'll be able to easily find and fix problems with your current cache policy. This will greatly improve performance for returning users, and they'll appreciate the extra speed!

Avoid Costly Multiple Round-Trips to Any Origin

When browsers retrieve resources from a server, it can take significant time to perform a DNS lookup and establish a connection to a server. rel=preconnect allows developers to mask this latency by establishing connections to other servers before the browser would in due course. The Avoid Costly Multiple Round-Trips to Any Origin audit will help you discover opportunities to use rel=preconnect!

When latency for cross-origin assets is reduced, users will perceive that things are moving along a bit quicker. With this new Lighthouse audit, you'll learn of new opportunities to use rel=preconnect to do just that.

Use Video Formats for Animated Content

Animated GIFs are huge, often consuming at least several hundred kilobytes if not several megabytes of data. If you care about loading performance, converting those GIFs to video is the way to go. Thankfully, the Use Video Formats for Animated Content audit has your back.

If your site has any GIFs that are over 100 KB, this audit will automatically flag them and direct you to some guidance on how to convert them to video and embed them. Sites like Imgur have significantly improved loading performance by converting their GIFs to video. Additionally, if your site is on a hosting plan with metered bandwidth, the potential cost savings alone should be enough to persuade you!

Give Lighthouse a try!

If you're excited about these new audits, update Lighthouse and give them a try!

• The Lighthouse Chrome extension should automatically update, but you can manually update it via chrome://extensions.
• In DevTools, you can run Lighthouse in the audits panel. Chrome updates to a new version about every 6 weeks, so some newer audits may not be available. If you're antsy to use the latest audits available, you can run the latest Chrome code by downloading Chrome Canary.
• For Node users: Run npm update lighthouse, or npm update lighthouse -g if you installed Lighthouse globally.