Well-deployed technologies
Timing Hooks
The Web Performance Working Group developed a number of specifications that expose timing hooks to Web applications, to analyze time spent doing various tasks.
The High-Resolution Time exposes a monotonic sub-millisecond resolution clock to Web applications so that they can precisely measure time elapsed between two events. The Performance Timeline defines a unified interface to store and retrieve performance metric data. Individual performance metric interfaces are enumerated in the Timing Entry Names Registry and defined in separate specifications:
- Navigation Timing exposes timing information related to navigation and elements;
- Resource Timing exposes timing information for resources in a document;
- User Timing help applications measure the performance of their applications using high precision timestamps.
Page Visibility Detection
The API to determine whether a Web page is being displayed (Page Visibility API) can also be used to adapt the usage of resources to the need of the Web application, for instance by reducing network activity when the page is minimized.
Priority Handling
The Cooperative Scheduling of Background Tasks specification defines the requestIdleCallback method that allows scheduling an operation at the next opportunity when the app is not processing another operation.
Low-level Bytecode Format
WebAssembly is a low-level bytecode format that runs with near-native speed in web browsers and supports compilation from C, C++, and other languages. It also defines an execution environment that attempts to maximize performance and interoperate gracefully with JavaScript and the Web, while ensuring security and consistent behavior across a variety of implementations.
Rendering performance
The CSS contain property can indicate that the element’s subtree is independent of the rest of the page. This also enables heavy optimizations by user agents when used well, in particular to skip over content that is off-screen knowing that it won't affect the rendering of the content that is on-screen.
Animation Optimization
The Timing control for script-based animations API can help reduce the usage of resources needed for playing animations.
Scrolling Optimization
Smooth scrolling performance is essential for a good user experience on the web, especially on touch-based devices. Through the passive event listener option, developers can declare up-front that an event listener will not call preventDefault() on the event, allowing the browser not to wait for the event listener to have run before it performs the default action associated with the event. This is particularly recommended on touch and wheel events to guarantee smooth scrolling. Note: some browsers automatically set the passive flag on touchstart and touchmove by default.
Threading
Beyond optimization of resources, the perceived reactivity of an application is also a critical aspect of the mobile user experience. The thread-like mechanism made possible via Web Workers allows keeping the user interface responsive by offloading the most resource-intensive operations into a background process.
Automation
The WebDriver specification defines a remote control interface that enables introspection and control of user agents, typically useful to automate testing across multiple browsers, including mobile browsers.
Optimization Best Practices
The Mobile Web Application Best Practices provide general advice on how to build Web applications that work well on mobile devices, taking into account in particular the needs for optimization.
Technologies in progress
Network Prioritization
The Resource Hints and Preload specifications let developers optimize the download of resources by enabling to delay either the download or the execution of the downloaded resource.
Caching
The Service Workers specification defines a mechanism that allows applications to intercept outgoing network requests and respond to them directly. Applications can take advantage of this mechanism to implement a flexible cache logic directly and thus avoid lengthy requests to the server.
Battery Status
The Battery status API allows adjusting the use of resources to the current level of power available in the battery of a mobile device. However, note the future of this last specification is uncertain due to identified potential privacy-invasive usage of the API.
Timing Hooks
Server Timing enables a server to communicate performance metrics about the request-response cycle to the user agent, and allows applications to act on these metrics to optimize application delivery.
The Long Tasks API exposes a mechanism to detect long running tasks that monopolize the user interface's main thread for extended periods of time.
The Paint Timing specification allows the application to capture a series of key moments such as first paint and first contentful paint during page load.
Rendering performance
To ensure optimal performance when animating parts of an app, developers can make use of the CSS will-change property to let browsers compute the animation ahead of its occurrence.
Animation Optimization
The CSS Animation Worklet API provides a method to create scripted animations that control a set of animation effects. The API is designed to make it possible for user agents to run such animations in their own dedicated thread to provide a degree of performance isolation from main thread.
Infinite scrolling
The use of infinite scrolling lists, where more and more content is loaded and rendered as the user scrolls, is very common on mobile devices. Such lists provide a better user experience than pagination on touch screens. Applications unfortunately need to continuously poll layout information of DOM elements synchronously to implement this pattern, which is a source of significant performance overhead. The Intersection Observer specification defines an API to asynchronously observe changes in the intersection of a target element with an ancestor element or with a top-level document's viewport, providing an efficient mechanism to retrieve the information needed to implement infinite scrolling.
Scrolling Optimization
User agents may implement default rules for scrolling such as scroll chaining and overscroll affordances that web applications may wish to disable to enhance pull-to-refresh and infinite scrolling interaction paradigms, which are common on mobile devices. This can be achieved through scripting, but negatively affects scrolling performances as the application needs to listen to touch events without setting the passive flag to override the default behavior when needed. The CSS overscroll-behavior property introduces control over the behavior of a scroll container when its scrollport reaches the boundary of its scroll box, allowing web applications to disable default rules for scrolling efficiently.
Real-time Communication
The Identifiers for WebRTC's Statistics API defines a set of Web IDL objects that allow access to the statistical information about a RTCPeerConnection, allowing web apps to monitor the performance of the underlying network and media pipeline.
Exploratory work
Timing hooks
The Frame Timing API aims at providing detailed information on the frame-per-second obtained when an application is running on the user device.
The Event Timing API exposes a mechanism to measure the latency of some events triggered by user interaction.
The Element Timing API enables monitoring when large or developer-specified image elements and text nodes are displayed on screen.
The Layout Instability API provides web page authors with insights into the stability of their pages based on movements of the elements on the page that detract from the user's experience.
Memory usage
The performance.measureMemory API proposal estimates the memory usage of a web page and provides a breakdown by type (JavaScript, DOM, specific objects, etc.) and owner (URL), so that web developers can perform statistical analysis of memory usage data of their application, catch memory leaks, and otherwise measure the memory impact of application features.
Network Prioritization
The Priority Hints specification lets developers signal the priority of each resource they need to download, complementing existing browser loading primitives such as preload.
DOM Updates
Web applications are dynamic in essence and need to manipulate the DOM significantly to present rich content. Problem is updates to the DOM can cause jank (noticeable delay in visual updates) because the rendering phase is updated synchronously with user interactions and requestAnimationFrame scripts. The Display Locking proposal introduces a new concept whereby developers can lock a DOM element and its subtree, preventing visual updates while the DOM gets updated. The developer will then be able to unlock the element, asynchronously, triggering the visual updates of the modified subtree without causing the rest of the page to jank.
Similarly, an early proposal to add requestPostAnimationFrame would give applications a hook into the earliest possible time after last frame has been rendered, and thus allow them to prepare for the next rendering update. The proposal could also be useful to run code at a time when the page layout is guaranteed to be clean, e.g. to avoid synchronous layouts when modifying a CSS property through code and then querying the size or position of the underlying element.
JS performance
Web applications have limited visibility into where JavaScript execution time is spent on clients. Without the ability to efficiently collect stack samples, applications are forced to instrument their code with profiling hooks that are imprecise and can significantly slow down execution. The JS Self-Profiling API proposal describes an API to manipulate a sampling profiler that would allow applications to gather rich execution data for aggregation and analysis of real end-user environments with minimal overhead.
Responsiveness
Applications that need to run long tasks (e.g. when pages are loaded) need to make a tradeoff between loading pages quickly (or reducing task execution duration) and responding to input quickly. The Early detection of input events specification proposes an isInputPending method that long running scripts can call synchronously, without losing time yielding to other scripts and events processing, to detect whether there are pending input events that their execution might delay from firing.
Applications that need to run long tasks may also want to regain control after yielding to the event loop before any other same or lower priority tasks are allowed to run. The scheduler.yield() function is being proposed to solve that issue.
Generally speaking, applications have little control over task priorities when they schedule them. The scheduler.postTask() proposal would create a unified API to schedule prioritized work.
Infinite scrolling
The creation of a built-in <virtual-scroller> element in HTML is under exploration in the Web Platform Incubator Community Group to avoid having to implement infinite scrolling purely in JavaScript. The browser would prioritize rendering children of that element that are in the viewport and not render children that are far away, thus improving performances while still allowing features such as "find in page", accessibility features, navigation focus, fragment URL navigation, etc. to work out of the box.