Web Performance

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Web performance is the objective measurements and the perceived user experience of load time and runtime. Web performance is how long a site takes to load, become interactive and responsive, and how smooth the content is during user interactions - is the scrolling smooth? are buttons clickable? Are pop-ups quick to open up, and do they animate smoothly as they do so? Web performance includes both objective measurements like time to load, frames per second, and time to interactive, and subjective experiences of how long it felt like it took the content to load.

The longer it takes for a site to respond, the more users will abandon the site. It is important to minimize the loading and response times, and add additional features to conceal latency by making the experience as available and interactive as possible, as soon as possible, while asynchronously loading in the longer tail parts of the experience.

There are tools, APIs, and best practices that help us measure and improve web performance. We cover them in this section

Key tutorials

The MDN Web Performance Learning Area contains modern, up-to-date tutorials covering Performance essentials:

What is web performance
This article starts the module off with a good look at what Performance actually is — this includes the tools, metrics, APIs, networks, and groups of people we need to consider when thinking about performance, and how we can make Performance part of our web development workflow.
HTML performance features
Some attributes and the source order of your markup can impact the performance or your website. By minimizing the number of DOM nodes, making sure the best order and attributes are used for included content such as styles, scripts, media, and third-party scripts, you can drastically improve the user experience. This article looks in detail at how HTML can be used to ensure maximum performance.
JavaScript performance best practices
JavaScript, when used properly, can allow for interactive and immersive web experiences ... or it can significantly harm download time, render time, in app performance, battery life, and user experience. This article outlines some JavaScript best practices that should be considered to ensure even complex content is as performant as possible.
Multimedia: Images and Video
The lowest hanging fruit of web performance is often media optimization. Serving different media files based on each user agent's capability, size, and pixel density is possible. Additional tips, like removing audio tracks from background images, can improve performance even further. In this article, we discuss the impact video, audio, and image content has on performance, and the methods to ensure that impact is as minimal as possible.
Advanced CSS performance features
CSS may be a less important optimization focus for improved performance, but there are some CSS features that impact performance more than others. In this article, we look at some CSS properties that impact performance and suggested ways of handling styles to ensure performance is not negatively impacted.
How do users perceive performance?

More important than how fast your website is in milliseconds, is how fast do your users perceive your site to be. These perceptions are impacted by actual page load time, idling, responsiveness to user interaction, and the smoothness of scrolling and other animations. In this article, we discuss the various loading metrics, animation, and responsiveness metrics, along with best practices to improve user perception, if not the actual timings

Web Performance Basics
In addition to the front end components of HTML, CSS, JavaScript, and media files, there are features that can make applications slower and features that can make applications subjectively and objectively faster. There are many APIs, Developer Tools, best practices and bad practices relating to web performance. Here we'll introduce many of these features ad the basic level and provide links to deeper dives to improve performance for each topic.
Mobile performance
With web access on mobile devices being so popular, and all mobile platforms having fully-fledged web browsers, but possibly limited bandwidth, CPU and battery life, it is important to consider the performance of your web content on these platforms. This article looks at mobile-specific performance considerations.
Populating the page
An HTTP request is made and, hopefully, a few seconds later, the site appears. Displaying the content involves executing JavaScript, possibly modifying the DOM, calculating styles, calculating layout, and finally rendering the content, which involves painting and compositing, and can involve GPU acceleration on a separate thread.
Performance bottlenecks
 
Understanding latency

Latency is the amount of time it takes between the browser making a request for a resource, and the browser receiving back the first byte of the resource requested. This article explains what latency is, how it impacts performance, and how to measure and improve latency.

Understanding bandwidth

Bandwidth is the amount of data (measured in Mbps or Kbps) that can be sent per second. This article explains the role of bandwidth in media-rich internet applications, how it can be measured, and how you can optimize applications to make the best use of available bandwidth.

HTTP/2 and you

The transport layer—that is, HTTP—is utterly essential to the functioning of the web, and it has only been relatively recently that it has seen a major update in the form of HTTP/2. Out of the box, HTTP/2 provides many performance improvements and advantages over its predecessor, but it also changes the landscape. In this article, you'll learn what HTTP/2 does for you, and how to fine-tune your application to make it do go even further.

The role of TLS in performance

TLS—or HTTPS as we tend to call it—is crucial in creating secure and safe user experiences. While hardware has reduced the negative impacts TLS has had on server performance, it still represents a substantial slice of the time we spend waiting for browsers to connect to servers. This article explains the TLS handshake process, and offers some tips for reducing this time, such as OCSP stapling, HSTS preload headers, and the potential role of resource hints in masking TLS latency for third parties.

Reading performance charts
Developer tools provide information on performance, memory, and network requests. Knowing how to read  waterfall charts, call trees, traces, flame charts , and allocations in your browser developer tools will help you understand waterfall and flame charts in other performance tools.
Analyzing JavaScript bundles
No doubt, JavaScript is a big part of modern web development. While you should always strive to reduce the amount of JavaScript you use in your applications, it can be difficult to know where to start. In this guide, we'll show you how to analyze your application's script bundles, so you know what you're using, as well how to detect if there are duplicated scripts between bundles in your app.
Lazy-loading JavaScript with dynamic imports
When developers hear the term "lazy loading", they immediately think of below-the-fold imagery that loads when it scrolls into the viewport. But did you know you can lazy load JavaScript as well? In this guide, we'll talk about the dynamic import() statement, which is a feature in modern browsers that loads a JavaScript module on demand. Of course, since this feature isn't available everywhere, we'll also show you how you can configure your tooling to use this feature in a widely compatible fashion.
Controlling resource delivery with resource hints
Browsers often know better than we do when it comes to resource prioritization and delivery—but they're far from clairyovant. Native browser features enable us to hint to the browser when it should connect to another server, or preload a resource before the browser knows it ever needs it. When used judiciously, this can make fast experience seem even faster. In this article, we cover native browser features like rel=preconnect, rel=dns-prefetch, rel=prefetch, and rel=preload, and how to use them to your advantage.

Other documentation

The business case for performance

You know web performance is important, but how do you convince clients and management to invest in performance and make it a priority. In this article, we'll discuss creating a clear business case to convince decision-makers to make the investment. 

Performance Budgets
Marketing, design, and sales needs, and developer experience, often ad bloat, third-party scripts, and other features that can slow down web performance. To help set priorities, it is often helpful to set a performance budget: a set of restrictions to not exceed during the development phase. In this article, we'll discuss creating and sticking to a performance budget. 
Mobile performance checklist
A concise checklist of performance considerations impacting mobile network users on hand-held, battery operated devices.
Optimizing Startup Performance 
How long does your app take to start up? Does it lock up the browser while loading? Take the time to ensure your app starts up nicely. This article offers tips and suggestions to help you achieve that goal
Developer Tools Performance Features
This section provides information on how to use and understand the performance features in your developer tools, including Waterfall, Call Tree, and Flame Charts.
Understanding Latency
Latency is the amount of time it takes between the browser making a request for a resource, and the browser receiving back the first byte of the resource requested. This article explains what latency is, how it impacts performance, and how to measure and improve latency.
Web performance checklist
A performance checklist of features to consider when developing applications with links to tutorials on how to implement each features, include service workers, diagnosing performance problems, font loading best practices, client hints, creating performant animations, etc.

App Performance

Performance fundamentals
A wide overview of Web application performance, what it is, how the browser helps to improve it, and what tools and processes you can use to test and further improve it.
Optimizing startup performance
Tips and suggestions to help you improve startup performance, both when writing a new app and when porting an app from another platform to the Web.
Profiling with the built-in profiler
Learn how to profile app performance with Firefox's built-in profiler.
CSS and JavaScript animation performance
Animations are critical for a pleasurable user experience. This article discusses the performance differences between CSS- and JavaScript-based animations. 

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See also

HTML

CSS

  • will-change
  • GPU v CPU
  • Measuring layout
  • font-loading best practices

JavaScript

APIs

Headers

Tools

Additional Metrics

  • Speed Index and Perceptual Speed Index

Best Practices

Document Tags and Contributors

Contributors to this page: christopherwilson1, chrisdavidmills, estelle
Last updated by: christopherwilson1,