Web Performance

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 load and display, 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 become 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 performance guides

웹페이지를 표시한다는 것: 브라우저는 어떻게 동작하는가

Users want web experiences with content that is fast to load and smooth to interact with. Therefore, a developer should strive to achieve these two goals.

중요 렌더링 경로

중요 렌더링 경로 (Critical Rendering Path)는 브라우저가 HTML, CSS, Javascipt를 화면에 픽셀로 변화하는 일련의 단계를 말하며 이를 최적화하는 것은 렌더링 성능을 향상시킵니다. 중요 렌더링 경로는 Document Object Model (DOM), CSS Object Model (CSSOM), 렌더 트리 그리고 레이아웃을 포함합니다.

Beginner's tutorials

The MDN Web Performance Learning Area contains modern, up-to-date tutorials covering Performance essentials. Start here if you are a newcomer to performance:

Web performance: brief overview

Overview of the web performance learning path. Start your journey here.

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.

How do users perceive performance?

More important than how fast your website is in milliseconds, is how fast 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.

HTML performance features

Some attributes and the source order of your mark-up can impact the performance or your website. By minimizing the number of DOM nodes, making sure the best order and attributes are used for including 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.

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 videos 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.

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.

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.

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.

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

Firefox Profiler Performance Features

This website provides information on how to use and understand the performance features in your developer tools, including Call Tree, Flame Graph, Stack Chart, Marker Chart and Network Chart.

Profiling with the built-in profiler

Learn how to profile app performance with Firefox's built-in profiler.

Glossary Terms

See also