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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

In this section

Critical rendering path
The Critical Rendering Path is the sequence of steps the browser goes through to convert the HTML, CSS, and JavaScript into pixels on the screen. Optimizing the critical render path improves render performance.The critical rendering path includes the Document Object Model (DOM), CSS Object Model (CSSOM), render tree and layout.
CSS and JavaScript animation performance
Browsers are able to optimize rendering flows. In summary, we should always try to create our animations using CSS transitions/animations where possible. If your animations are really complex, you may have to rely on JavaScript-based animations instead.
Navigation and resource timings
Navigation timing and resource timing are metrics measuring a browser's document navigation events and detailed network timing data regarding the loading of an application's resources respectively.
Optimizing startup performance
Regardless of platform, it's always a good idea to start up as quickly as possible. Since that's a universal issue, we won't be focusing on it too much here. Instead, we're going to look at a more important issue when building Web apps: starting up as asynchronously as possible. That means not running all your startup code in a single event handler on the app's main thread.
Performance budgets
A performance budget is a limit to prevent regressions. It can apply to a file, all files loaded on a page, a specific metric (e.g Time to Interactive), a custom metric (e.g Time to Hero Element), or a threshold over a period of time.   
Performance fundamentals
Performance means efficiency. In the context of Open Web Apps, this document explains in general what performance is, how the browser platform helps improve it, and what tools and processes you can use to test and improve it.
Performance Monitoring: RUM vs synthetic monitoring
Sythentic monitoring and real user monitoring (RUM) are two approaches for monitoring and providing insight into web performance. In this article we define and compare these two performance monitoring approaches.
Populating the page: how browsers work
a developer's goal should be to send requested resources as fast as possible, and ensure user interactions are smooth. To understand how to improve performance and perceived performance, it helps to understand how the browser works.
The business case for web 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.

Selected 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.
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.
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. Page load time, idling, responsiveness to user interaction, and the smoothness of scrolling and other animations impact these perceptions. 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.

Multimedia: Images and Video
Frequently, media optimization is the lowest hanging fruit of web performance. 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.
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.

Using Performance APIs

Performance API
This guide describes how to use the Performance interfaces that are defined in the High-Resolution Time standard.
Resource Timing API
Resource loading and timing the loading of those resources, including managing the resource buffer and coping with CORS
The performance timeline
The Performance Timeline standard defines extensions to the Performance interface to support client-side latency measurements within applications. Together, these interfaces can be used to help identify an application's performance bottlenecks.
User Timing API
Create application specific timestamps using the user timing API's "mark" and "measure" entry types - that are part of the browser's performance timeline.
Frame Timing API
The PerformanceFrameTiming interface provides frame timing data about the browser's event loop.
Beacon API
The Beacon interface schedules an asynchronous and non-blocking request to a web server.
Intersection Observer API
Learn to time element visibility with the Intersection Observer API and be asynchronously notified when elements of interest becomes visible.

Other documentation

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, how to measure latency, and how to reduce it.

Glossary Terms

Documents yet to be written

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 can ensure even complex content is as performant as possible.
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.
Performance bottlenecks
Understanding bandwidth
Bandwidth is the amount of data (measured in Mbps or Kbps) that one can send per second. This article explains the role of bandwidth in media-rich internet applications, how you can measure it, 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 your app contains duplicated scripts between bundles.
Lazy loading
It isn't always necessary to load all of a web applications assets on initial page load. Lazy Loading is defering the loading of assets on a page, such as scripts, images, etc., on a page to a later point in time – when those assets are actually needed.
Lazy-loading JavaScript with import()
The term "lazy loading" often refers to load deferment techniques for assets not needed at load such as loading below-the-fold imagery only when those images scroll into the view. Now there are native features to load JavaScript! In this guide, we'll talk about the dynamic import() statement, a newer browser feature, that loads a JavaScript module on demand.
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.
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 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. 
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.
Mobile performance checklist
A concise checklist of performance considerations impacting mobile network users on hand-held, battery operated devices.

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. 

View all articles about performance...

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

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