Optimize website speed using Lighthouse

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

This tutorial uses the Lighthouse tool. The Lighthouse tool provides links to content hosted on third-party websites. Microsoft is not responsible for and has no control over the content of these sites and any data that may be collected.

The Lighthouse tool was previously called the Audits tool. "Audits panel" means the same thing as "Lighthouse tool".


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

  • You don't need to know anything about load performance. You learn about load performance in this tutorial.


This is Tony. Tony is very famous in cat society. He has built a website so that his fans can learn about his favorite foods. 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 the load performance of a site, 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 have the most impact.


First, you must set up the site so that you can make changes to it later.

  1. Open the source code for the site. This tab is referred to as the editor tab.

    The editor tab

  2. Click tony. A menu appears.

    The menu that appears after clicking 'tony'

  3. Click Remix Project. The name of the project changes from tony to a randomly generated name. You now have your own editable copy of the code. Later on, you can make changes to this code.

  4. Click Show and then select In a New Window. The demo opens in a new tab. This tab is be referred to as the demo tab. It may take a while for the site to load.

    The demo tab

  5. Press Ctrl+Shift+J (Windows, Linux) or Command+Option+J (macOS). DevTools opens alongside the rendered demo webpage.

    DevTools and the demo

For the rest of the screenshots in this tutorial, DevTools is shown in a separate window. Press Ctrl+Shift+P (Windows, Linux) or Command+Shift+P (macOS) to open the Command Menu, type Undock, and then select Undock into separate window.

DevTools undocked into a separate window

Establish a baseline

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

  1. Select the Lighthouse tool. It might be hidden behind the More tools (More tools) button.

    The Lighthouse tool

  2. Match your audit configuration settings to those in the previous figure. Here is an explanation of the different options:

    • Device. Set to Mobile changes the user agent string and simulates a mobile viewport. Set to Desktop pretty much just turns off the Mobile changes.

    • Audits. Turn off a category to prevent the Audits panel from running those audits, and excludes those audits from your report. Leave the other categories Turned on, if you want to display the types of recommendations that are provided. Turn off categories to slightly speed up the auditing process.

    • Throttling. Set to Simulated Slow 4G, 4x CPU Slowdown simulates the typical conditions of browsing on a mobile device. It is named "simulated" because the Audits panel doesn't actually throttle during the auditing process. Instead, it just extrapolates how long the page takes to load under mobile conditions. The Applied... setting, on the other hand, actually throttles your CPU and network, with the tradeoff of a longer auditing process.

    • Clear Storage. Turn on the checkbox to clear 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. Turn off this setting when you want the repeat-visit experience.

  3. Click Run Audits. After 10 to 30 seconds, the Audits panel displays a report of the performance of the site.

    The report for the Audits panel of the performance of the site

Handling report errors

If you ever get an error in your Audits panel report, try running the demo tab from an InPrivate window with no other tabs open. This ensures that you are running Microsoft Edge from a clean state. Microsoft Edge Extensions in particular often interfere with the auditing process.

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, the number displayed should rise. A higher score means better performance.

The overall performance score

The Metrics section provides quantitative measurements of the performance of the site. 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.

The Metrics section

Click the highlighted toggle button in the following figure to display a description for each metric. Then click Learn More to read documentation about it.

Click the highlighted toggle button to expand the Metrics items

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

How the page looked while loading

The Opportunities section provides specific tips on how to improve the load performance of this specific page.

The Opportunities section

Click an opportunity to learn more about it.

Eliminate render-blocking resources opportunity

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

Documentation for the Eliminate render-blocking resources opportunity

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

The Diagnostics section

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 audit report gives you tips on how to improve the performance of the page. 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 avoiding enormous network payloads is one of the top opportunities for improving the performance of the page. Enabling text compression is an opportunity to improve the performance of the page.

Text compression is when you reduce, or compress, the size of a text file before sending it over the network. This compression is similar to how you can archive a directory before sending it, to reduce the size.

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

  1. Click the Network tool.

    The Network panel

  2. Click the Network setting icon.

  3. Click the Use Large Request Rows checkbox. The height of the rows in the table of network requests increases.

    Large rows in the network requests table

  4. If the Size column in the table of network requests isn't displayed, click the table header > Size.

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 isn't being compressed when it is sent over the network. For example, in the previous figure, the top and bottom values for bundle.js are 1.2 MB and 1.2 MB.

Check for compression by inspecting the HTTP headers of a resource:

  1. Click bundle.js.

  2. Click the Headers panel.

    The Headers panel

  3. Search the Response Headers section for a content-encoding header. A content-encoding heading isn't displayed, meaning that bundle.js wasn't compressed. When a resource is compressed, this header is usually set to gzip, deflate, or br. For an explanation of the values, see Directives.

Enough with the explanations; it's time to make some changes. Enable text compression by adding a couple of lines of code, as follows.

  1. In the editor tab, select server.js.

    Edit server.js

  2. Add the following code to server.js. Make sure to put app.use(compression()) before app.use(express.static('build')).

    const express = require('express');
    const app = express();
    const fs = require('fs');
    const compression = require('compression');
    const listener = app.listen(process.env.PORT || 1234, function () {
       console.log(`Listening on port ${listener.address().port}`);


    Usually, you have to install the compression package via something like npm i -S compression, but this has already been done for you.

  3. Wait for Glitch to deploy the new build of the site. The fancy animation next to Tools means that the site is getting rebuilt and redeployed. The change is ready when the animation next to Tools goes away. Click Show and then select In a New Window again.

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

  1. Go back to the demo tab and refresh the page. The Size column should now show 2 different values for text resources like bundle.js. In the figure after the following, the top value of 256 KB for bundle.js is the size of the file that was sent over the network, and the bottom value of 1.2 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 now includes a content-encoding: gzip header:

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

Audit the page again to measure what kind of impact text compression has on the load performance of the page:

  1. Select the Lighthouse tool (formerly called the Audits tool).

  2. Click Perform an audit (Perform an audit).

  3. Keep the settings the same as before.

  4. Click Run audit.

    An Audits report after enabling text compression

Your overall performance score should have increased, meaning that the site is getting faster.

Text compression in the real world

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

Resize images

Your report indicates that avoiding enormous network payloads is one of the top opportunities for improving the performance of the page. Resizing images helps reduce the size of the network payload. If your user is viewing your images on a mobile device screen that is 500-pixels-wide, there is really no point in sending a 1500-pixel-wide image. Ideally, you send a 500-pixel-wide image, at most.

  1. In your report, select Avoid enormous network payloads to display which images should be resized. It looks like two of the .jpg files are over 2000 KB, which is bigger than necessary.

  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 the change affects load performance.

    An Audits report after resizing images

The change only has a minor effect 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 5.3 megabytes, whereas now it's only about 0.18 megabytes.

Resizing images in the real world

For a small app, doing a one-off resize like this might be good enough. But for a large app, this 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 which size is best for the device.

  • 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. For more tips, see Essential Image Optimization.

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 run before it displays 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 you don't need to run on page load.

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

    More information about the Eliminate render-blocking resources opportunity

  2. Press Ctrl+Shift+P (Windows, Linux) or Command+Shift+P (macOS) to open the Command Menu, start typing Coverage, and then select Show Coverage.

    Open the Command Menu from the Audits panel

    The Coverage tool

  3. Click Refresh (Refresh). The Coverage tool provides an overview of how much of the code in bundle.js, jquery.js, and lodash.js runs while the page loads. In the figure after the following, about 76% and 30% of the jQuery and Lodash files aren't used, respectively.

    The Coverage report

  4. Click the jquery.js row. DevTools opens the file in the Sources tool. If a line of code ran, a blue bar appears next to it. A red bar means the line of code was not run, and is definitely not needed on load of the webpage.

    Viewing the jQuery file in the Sources tool

  5. Scroll through the jQuery code. Some of the lines that run are actually just comments. To strip comments and reduce the size of the file, run the code through a minifier app or script.

In short, when you're working with your own code, the Coverage tool helps 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 tool shows what happens when resources aren't available:

  1. Select the Network tool.

  2. Press Ctrl+Shift+P (Windows, Linux) or Command+Shift+P (macOS) to open the Command Menu again.

  3. Start typing blocking, and then select Show Request Blocking.

    The Request blocking tool

  4. Click Add Pattern (Add Pattern), type /libs/*, and then press Enter to confirm.

    Add a pattern to block any request to the libs directory

  5. Refresh the page. The jQuery and Lodash requests are red, meaning that the requests 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

  6. Click Remove all patterns (Remove all patterns) to delete the /libs/* blocking pattern.

In general, the Request blocking tool 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 <script src="/libs/lodash.js"> and <script src="/libs/jquery.js"></script>.

  3. Wait for the site to re-build and re-deploy.

  4. Audit the page again from the Audits tool. Your overall score should have improved again.

    An Audits 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, speed up page load by only shipping critical code during the page load, and then lazy-loading everything else.

  • It's unlikely that you can find scripts that you can remove outright, but you might find many scripts that you don't need to request during the page load, which can instead be requested asynchronously.

  • If you're using a framework, check whether it has a production mode. This production mode might use a feature such as tree shaking in order to eliminate unnecessary code that's blocking the critical render.

Do less main thread work

Your latest report shows some minor potential savings in the Opportunities section, but if you look down in 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 running 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. Select the Performance tool.

  2. Click Capture Settings (Capture Settings).

  3. Set Network to Slow 3G and CPU to 6x slowdown. 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.

  4. Click Refresh (Refresh). DevTools refreshes the page and then produces a visualization of all the work performed in order to load the page. This visualization is referred to as the trace.

    The Performance tool 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 block of yellow highlighted in the figure after the next, 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.

The Overview section of the trace

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

  1. Click the Timings section to expand it. Based on the fact that there may be a bunch of Timing measures from React, it seems like Tony's app is using the development mode of React. Switching to the production mode of React may yield some easy performance wins.

    The Timings section

  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. For example, in the figyre after the following, the Evaluate Script event caused the (anonymous) function to run, which caused (anonymous) to run, which caused __webpack__require__ to run, and so on.

The Main section

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

    Hover on the mineBitcoin activity


    Although the requests that your framework makes are usually out of your control, sometimes you might structure your app in a way that causes the framework to run inefficiently. Restructuring your app to use the framework efficiently is a way to do less main thread work. However, this requires a deep understanding of how your framework works, and what kind of changes you make in your own code in order to use the framework more efficiently.

  2. Expand the Bottom-Up section. This tab breaks down what activities took up the most time. If nothing is displayed in the Bottom-Up section, click the label for Main section. The Bottom-Up section only shows information for whatever activity, or group of activity, you have currently selected. For example, if you chose one of the mineBitcoin activities, the Bottom-Up section is only going to show information for that one activity.

    The Bottom-Up tab

The Self Time column shows you how much time was spent directly in each activity. For example, in the following figure, about 63% of main thread time was spent on the mineBitcoin function.

It's time to see whether using production mode and reducing JavaScript activity may speed 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.

    An Audits report after configuring webpack to use production mode

Reduce JavaScript activity by removing the request to mineBitcoin:

  1. In the editor tab, open src/App.jsx.

  2. Comment out the request to this.mineBitcoin(1500) in the constructor.

  3. Wait for the new build to deploy.

  4. Audit the page again.

    An Audits report after removing unnecessary JavaScript work

Looks like that last change caused a massive jump in performance!

This section provided a rather brief introduction to the Performance tool. To learn more about how to analyze page performance, see Performance features reference).

Doing less main thread work in the real world

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

If you prefer an approach that feels more like console.log(), the User Timing API enables you to 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 the load performance of a site, 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 webpage after each change in order to display how that isolated change affects performance.


Portions of this page are modifications based on work created and shared by Google and used according to terms described in the Creative Commons Attribution 4.0 International License. The original page is found here and is authored by Kayce Basques (Technical Writer, Chrome DevTools & Lighthouse).

Creative Commons License This work is licensed under a Creative Commons Attribution 4.0 International License.