Lighthouse: Optimize website speed

Sofia Emelianova
Sofia Emelianova

Goal of tutorial

This tutorial teaches you how to use Chrome DevTools to find ways to make your websites load faster.

Read on, or watch the video version of this tutorial:


You should have basic web development experience, similar to what's taught in this Introduction to Web Development class.

You don't need to know anything about load performance.


This is Tony. Tony is very famous in cat society. He has built a website so that his fans can learn what his favorite foods are. His fans love the site, but Tony keeps hearing complaints that the site loads slowly. Tony has asked you to help him speed the site up.

Tony the cat.

Step 1: Audit the site

Whenever you set out to improve a site's load performance, always start with an audit. The audit has two important functions:

  • It creates a baseline for you to measure subsequent changes against.
  • It gives you actionable tips on what changes will have the most impact.

Set up

First, you need to set up a new working environment for Tony's website, so that you can make changes to it later:

  1. Remix the website's project on Glitch. Your new project opens in a tab. This tab will be referred to as the editor tab.

    The original source and the editor tab after clicking Remix.

    The name of the project changes from tony to some randomly-generated name. You now have your own editable copy of the code. Later on, you'll make changes to this code.

  2. At the bottom of the editor tab, click Preview > Preview in a new window. The demo opens in a new tab. This tab will be referred to as the demo tab. It may take a while for the site to load.

    The demo tab.

  3. Open DevTools alongside the demo.

    DevTools and the demo.

Establish a baseline

The baseline is a record of how the site performed before you made any performance improvements.

  1. Open the Lighthouse panel. It may be hidden behind More panels.

    The Lighthouse panel.

  2. Match your Lighthouse report configuration settings to those on the screenshot. Here's an explanation of the different options:

    • check_box Clear Storage. Enabling this checkbox clears all storage associated with the page before every audit. Leave this setting on if you want to audit how first-time visitors experience your site. Disable this setting when you want the repeat-visit experience.
    • check_box Enable JS sampling. This option is turned off by default. When turned on, it adds detailed JavaScript call stacks to the performance trace but may slow down report generation. The trace is available under more_vert Tools menu > View Unthrottled Trace after the Lighthouse report is generated. Performance trace without (left) and with (right) JS sampling.
    • Simulated throttling (default) . This option simulates the typical conditions of browsing on a mobile device. It's called "simulated" because Lighthouse doesn't actually throttle during the auditing process. Instead, it just extrapolates how long the page would take to load under mobile conditions. The DevTools throttling (advanced) setting, on the other hand, actually throttles your CPU and network, with the tradeoff of a longer auditing process.
    • Mode > Navigation (Default). This mode analyses a single page load and that's what we need in this tutorial. For more information, see The three modes.
    • Device > Mobile. The mobile option changes the user agent string and simulates a mobile viewport. The desktop option pretty much just disables the mobile changes.
    • Categories > check_box Performance. A single enabled category makes Lighthouse generate a report only with the corresponding set of audits. You can leave the other categories enabled, if you want to see the types of recommendations they provide. Disabling irrelevant categories slightly speeds up the auditing process.
  3. Click Analyze page load. After 10 to 30 seconds, the Lighthouse panel shows you a report of the site's performance.

    A Lighthouse report of the site's performance.

Handling report errors

If you ever get an error in your Lighthouse report, try running the demo tab from an incognito window with no other tabs open. This ensures that you're running Chrome from a clean state. Chrome Extensions in particular can interfere with the auditing process.

A report with an error.

Understand your report

The number at the top of your report is the overall performance score for the site. Later, as you make changes to the code, you should see this number rise. A higher score means better performance.

The overall performance score.


Scroll down to the Metrics section and click Expand view. To read documentation on a metric, click Learn more....

The Metrics section.

This section provides quantitative measurements of the site's performance. Each metric provides insight into a different aspect of the performance. For example, First Contentful Paint tells you when content is first painted to the screen, which is an important milestone in the user's perception of the page load, whereas Time To Interactive marks the point at which the page appears ready enough to handle user interactions.


Next is a collection of screenshots that show you how the page looked as it loaded.

Screenshots of how the page looked while loading.


Next is the Opportunities section that provides specific tips on how to improve this particular page's load performance.

The Opportunities section.

Click an opportunity to learn more about it.

More information about the text compression opportunity.

Click Learn more... to see documentation about why an opportunity is important, and specific recommendations on how to fix it.


The Diagnostics section provides more information about factors that contribute to the page's load time.

The Diagnostics section.

Passed audits

The Passed audits section shows you what the site is doing correctly. Click to expand the section.

The Passed audits section.

Step 2: Experiment

The Opportunities section of your Lighthouse report gives you tips on how to improve the page's performance. In this section, you implement the recommended changes to the codebase, auditing the site after each change to measure how it affects site speed.

Enable text compression

Your report says that enabling text compression is one of the top opportunities for improving the page's performance.

Text compression is when you reduce, or compress, the size of a text file before sending it over the network. Kind of like how you might zip a folder before emailing it to reduce its size.

Before you enable compression, here are a couple of ways you can manually check whether text resources are compressed.

Open the Network panel and check Settings > Use large request rows.

The Size column in the Network panel showing large request rows.

Each Size cell shows two values. The top value is the size of the downloaded resource. The bottom value is the size of the uncompressed resource. If the two values are the same, then the resource is not being compressed when it's sent over the network. In this example, the top and bottom values for bundle.js are both 1.4 MB.

You can also check for compression by inspecting a resource's HTTP headers:

  1. Click bundle.js and open the Headers tab.

    The Headers tab.

  2. Search the Response Headers section for a content-encoding header. You shouldn't see one, meaning that bundle.js was not compressed. When a resource is compressed, this header is usually set to gzip, deflate, or br. See Directives for an explanation of these values.

Enough with the explanations. Time to make some changes! Enable text compression by adding a couple of lines of code:

  1. In the editor tab, open server.jsand add the following two (highlighted) lines:

    const fs = require('fs');
    const compression = require('compression');
  2. Make sure to put app.use(compression()) before app.use(express.static('build')).

    Editing server.js.

  3. Wait for Glitch to deploy the new build of the site. A happy emoji in the bottom left corner indicates a successful deployment.

Use the workflows that you learned earlier to manually check that the compression is working:

  1. Go back to the demo tab and reload the page.

    The Size column should now show two different values for text resources like bundle.js. The top value of 269 KB for bundle.js is the size of the file that was sent over the network, and the bottom value of 1.4 MB is the uncompressed file size.

    The Size column now shows two different values for text resources.

  2. The Response Headers section for bundle.js should now include a content-encoding: gzip header.

    The Response Headers section now contains a content-encoding header.

Run the Lighthouse report on the page again to measure the impact the text compression has on the page's load performance:

  1. Open the Lighthouse panel and click Add. Perform an audit... on the action bar at the top.

    The Perform an audit button.

  2. Leave the settings the same as before and click Analyze page load.

    A Lighthouse report after enabling text compression.

Woohoo! That looks like progress. Your overall performance score should have increased, meaning that the site is getting faster.

Text compression in the real world

Most servers really do have simple fixes like this for enabling compression! Just do a search on how to configure whatever server you use to compress text.

Resize images

Your new report says that properly sizing images is another big opportunity.

Resizing images helps speed up load time by reducing the file size of images. If your user is viewing your images on a mobile device screen that's 500-pixels-wide, there's really no point in sending a 1500-pixel-wide image. Ideally, you'd send a 500-pixel-wide image, at most.

  1. In your report, click Properly size images to see what images should be resized. It looks like all 4 images are bigger than necessary.

    Details about the 'properly size images' opportunity

  2. Back in the editor tab, open src/model.js.

  3. Replace const dir = 'big' with const dir = 'small'. This directory contains copies of the same images which have been resized.

  4. Audit the page again to see how this change affects load performance.

    A Lighthouse report after resizing images.

Looks like the change only has a minor affect on the overall performance score. However, one thing that the score doesn't show clearly is how much network data you're saving your users. The total size of the old photos was around 6.1 MB, whereas now it's only about 633 kB. You can check this on the status bar at the bottom of the Network panel.

Amount of data transferred before and after resizing images.

Resizing images in the real world

For a small app, doing a one-off resize like this may be good enough. But for a large app, this obviously isn't scalable. Here are some strategies for managing images in large apps:

  • Resize images during your build process.
  • Create multiple sizes of each image during the build process and then use srcset in your code. At runtime, the browser takes care of choosing what size is best for the device it's running on. See Relative-sized images.
  • Use an image CDN that lets you dynamically resize an image when you request it.
  • At the very least, optimize each image. This can often create huge savings. Optimization is when you run an image through a special program that reduces the size of the image file. See Essential Image Optimization for more tips.

Eliminate render-blocking resources

Your latest report says that eliminating render-blocking resources is now the biggest opportunity.

A render-blocking resource is an external JavaScript or CSS file that the browser must download, parse, and execute before it can show the page. The goal is to only run the core CSS and JavaScript code that is required to display the page properly.

The first task, then, is to find code that doesn't need to be executed on page load.

  1. Click Eliminate render-blocking resources to see the resources that are blocking: lodash.js and jquery.js.

    More information about the 'reduce render-blocking resources' opportunity.

  2. Depending on your operating system, press the following to open the Command Menu:

    • On Mac, Command+Shift+P
    • On Windows, Linux, or ChromeOS, Control+Shift+P
  3. Start typing Coverage and select Show Coverage.

    Opening the Command Menu from the Lighthouse panel.

    The Coverage tab opens in the Drawer.

    The Coverage tab.

  4. Click refresh Reload. The Coverage tab provides an overview of how much of the code in bundle.js, jquery.js, and lodash.js is being executed while the page loads.

    The Coverage report.

    This screenshot says that about 74% and 30% of the jQuery and Lodash files aren't used, respectively.

  5. Click the jquery.js row. DevTools opens the file in the Sources panel. A line of code was executed if it has a green bar next to it. A red bar next to a line of code means it was not executed, and is definitely not needed on page load.

    Viewing the jQuery file in the Sources panel.

  6. Scroll through the jQuery code a bit. Some of the lines that get "executed" are actually just comments. Running this code through a minifier that strips comments is another way to reduce the size of this file.

In short, when you're working with your own code, the Coverage tab can help you analyze your code, line-by-line, and only ship the code that's needed for page load.

Are the jquery.js and lodash.js files even needed to load the page? The Request Blocking tab can show you what happens when resources aren't available.

  1. Click the Network tab and open the Command Menu again.
  2. Start typing blocking and then select Show Request Blocking. The Request Blocking tab opens.

    The Request Blocking tab.

  3. Click Add. Add Pattern, type /libs/* in the textbox, and press Enter to confirm.

    Adding a pattern to block any request to the 'libs' directory.

  4. Reload the page. The jQuery and Lodash requests are red, meaning that they were blocked. The page still loads and is interactive, so it looks like these resources aren't needed whatsoever!

    The Network panel shows that the requests have been blocked.

  5. Click Remove. Remove all patterns to delete the /libs/* blocking pattern.

In general, the Request Blocking tab is useful for simulating how your page behaves when any given resource isn't available.

Now, remove the references to these files from the code and audit the page again:

  1. Back in the editor tab, open template.html.
  2. Delete the corresponding <script> tags:

        <meta name="viewport" content="width=device-width, initial-scale=1">
        <script src="/libs/lodash.js"></script>
        <script src="/libs/jquery.js"></script>
        <title>Tony's Favorite Foods</title>
  3. Wait for the site to re-build and re-deploy.

  4. Audit the page again from the Lighthouse panel. Your overall score should have improved again.

    A Lighthouse report after removing the render-blocking resources.

Optimizing the Critical Rendering Path in the real-world

The Critical Rendering Path refers to the code that you need to load a page. In general, you can speed up page load by only shipping critical code during the page load, and then lazy-loading everything else.

  • It's unlikely that you'll find scripts that you can remove outright, but you will often find that many scripts don't need to be requested during the page load, and can instead be requested asynchronously. See Using async or defer.
  • If you're using a framework, check if it has a production mode. This mode may use a feature such as tree shaking in order to eliminate unnecessary code that is blocking the critical render.

Do less main thread work

Your latest report shows some minor potential savings in the Opportunities section, but if you scroll down to the Diagnostics section, it looks like the biggest bottleneck is too much main thread activity.

The main thread is where the browser does most of the work needed to display a page, such as parsing and executing HTML, CSS, and JavaScript.

The goal is to use the Performance panel to analyze what work the main thread is doing while the page loads, and find ways to defer or remove unnecessary work.

  1. Open Performance > Settings. Capture Settings and set Network to Slow 3G and CPU to 6x slowdown.

    Settings CPU and network throttling in the Performance panel

    Mobile devices typically have more hardware constraints than laptops or desktops, so these settings let you experience the page load as if you were using a less powerful device.

  2. Click refresh Reload. DevTools reloads the page and then produces a visualization of all it had to do in order to load the page. This visualization will be referred to as the trace.

    The Performance panel's trace of the page load.

The trace shows activity chronologically, from left to right. The FPS, CPU, and NET charts at the top give you an overview of frames per second, CPU activity, and network activity.

The Overview section of the trace.

The wall of yellow that you see in the Overview section means that the CPU was completely busy with scripting activity. This is a clue that you may be able to speed up page load by doing less JavaScript work.

Investigate the trace to find ways to do less JavaScript work:

  1. Click the Timings section to expand it.

    The Timings section.

    There is a bunch of User Timing measures from React, it seems like Tony's app is using the development mode of React. Switching to the production mode of React will probably yield some easy performance wins.

  2. Click Timings again to collapse that section.

  3. Browse the Main section. This section shows a chronological log of main thread activity, from left to right. The y-axis (top to bottom) shows why events occurred.

    The Main section.

    In this example, the Evaluate Script event caused the (anonymous) function to execute, which caused __webpack__require__ to execute, which caused ./src/index.jsx to execute, and so on.

  4. Scroll down to the bottom of the Main section. When you use a framework, most of the upper activity is caused by the framework, which is usually out of your control. The activity caused by your app is usually at the bottom.

    The mineBitcoin activity.

    In this app, it seems like a function called App is causing a lot of calls to a mineBitcoin function. It sounds like Tony might be using the devices of his fans to mine cryptocurrency...

  5. Open the Bottom-Up tab at the bottom. This tab breaks down what activities took up the most time. If you don't see anything in the Bottom-Up section, click the label for Main section.

    The Bottom-Up tab.

    The Bottom-Up section only shows information for whatever activity, or group of activity, you have currently selected. For example, if you clicked on one of the mineBitcoin activities, the Bottom-Up section is only going to show information for that one activity.

    The Self Time column shows you how much time was spent directly in each activity. In this case, about 82% of main thread time was spent on the mineBitcoin function.

Time to see whether using production mode and reducing JavaScript activity speeds up the page load. Start with production mode:

  1. In the editor tab, open webpack.config.js.
  2. Change "mode":"development" to "mode":"production".
  3. Wait for the new build to deploy.
  4. Audit the page again.

    A Lighthouse report after configuring webpack to use production mode.

Reduce JavaScript activity by removing the call to mineBitcoin:

  1. In the editor tab, open src/App.jsx.
  2. Comment out the call to this.mineBitcoin(1500) in the constructor.
  3. Wait for the new build to deploy.
  4. Audit the page again.

A Lighthouse report after removing unnecessary JavaScript work.

As always, there still are things to do, for example, reduce the Largest Contentful Paint and Cumulative Layout Shift metrics.

Doing less main thread work in the real world

In general, the Performance panel is the most common way to understand what activity your site does as it loads, and find ways to remove unnecessary activity.

If you'd prefer an approach that feels more like console.log(), the User Timing API lets you arbitrarily mark up certain phases of your app lifecycle, in order to track how long each of those phases takes.


  • Whenever you set out to optimize a site's load performance, always start with an audit. The audit establishes a baseline, and gives you tips on how to improve.
  • Make one change at a time, and audit the page after each change in order to see how that isolated change affects performance.

Next steps

Run audits on your own site! If you need help interpreting your report or finding ways to improve your load performance, check out all the ways to get help from the DevTools community:

  • File bugs on this doc in the repository.
  • File bug reports on DevTools at Chromium Bugs.
  • Discuss features and changes on the Mailing List. Please don't use the mailing list for support questions. Use Stack Overflow, instead.
  • Get general help on how to use DevTools on Stack Overflow. To file bug requests, always use Chromium Bugs.
  • Tweet us at @ChromeDevTools.