Performance & Core Web Vitals

Core Web Vitals are three metrics Google uses to describe the user experience of a web page.

Cumulative Layout Shift (CLS) is a Core Web Vitals metric for visual stability. web.dev defines it as the largest burst of layout shift …

First Contentful Paint (FCP) is a performance metric for the time between the start of navigation and the moment the first piece of …

First Input Delay (FID) was a Core Web Vitals metric.

Interaction to Next Paint (INP) is a Core Web Vitals metric for how quickly a page responds.

Largest Contentful Paint (LCP) is a Core Web Vitals metric for loading speed.

Page experience is Google's term for the signals that show how pleasant a page is for visitors.

Speed Index is a lab metric for page speed.

Time to First Byte (TTFB) is the time between the start of navigation to a page and the moment the first byte of the response arrives. …

Total Blocking Time (TBT) is a lab metric.

The Chrome UX Report (CrUX) is a public Google dataset that reflects how real Chrome users experience websites.

Field data is performance data collected from real visitors on their own devices and networks.

Lighthouse is, in the web context, an open-source, automated tool from Google that audits the quality of web pages.

Page speed is how quickly a web page loads, becomes visible, and responds to user input.

PageSpeed Insights (PSI) is a free Google tool that analyzes the performance of a web page on mobile and desktop and suggests improvements.

Real user monitoring (RUM) collects performance data from real visits to a website or app, using code that runs in the visitor's browser.

In web development, web performance is how quickly a website loads, becomes usable and reacts to input.

The back/forward cache (bfcache) is a browser optimization that keeps a complete snapshot of a page in memory when a user navigates away.

HTTP compression is the process of shrinking text files such as HTML, CSS and JavaScript on the server before they travel over the network.

Critical CSS is the CSS needed to render the content that is visible without scrolling (above the fold).

DOM size is the total number of elements in the Document Object Model of a web page, the in-memory tree the browser builds from the HTML.

fetchpriority is an HTML attribute that tells the browser how important a resource is compared to other resources of the same type.

Minification is the process of removing unnecessary or redundant data from a resource without changing how the browser processes it (MDN).

Preload, preconnect and prefetch are resource hints.

The Speculation Rules API lets a web page tell the browser which URLs a visitor will probably open next.

Third-party scripts are JavaScript files that a website loads from domains it does not control.

Web font loading describes how a page fetches custom font files and what the browser shows while they download.