Web Performance Glossary

Core Web Vitals

Google's three user experience metrics for loading (LCP), responsiveness (INP) and visual stability (CLS), each judged at the 75th percentile of page views. See Core Web Vitals for Casino Websites.

Largest Contentful Paint (LCP)

The time from the start of navigation until the largest image or text block in the viewport renders. Good is 2.5 seconds or less. See Largest Contentful Paint (LCP).

Interaction to Next Paint (INP)

A measure of how quickly a page responds to clicks, taps and key presses across a visit, usually reported as the slowest interaction. Good is 200 milliseconds or less. See Interaction to Next Paint (INP).

Cumulative Layout Shift (CLS)

The largest burst of unexpected layout shift scores during a visit. Good is 0.1 or less. See Cumulative Layout Shift (CLS).

First Input Delay (FID)

The former responsiveness metric. INP replaced it as a Core Web Vital on March 12, 2024.

Time to First Byte (TTFB)

The time until the browser receives the first byte of the page response. It is the first of the four parts of LCP.

First Contentful Paint (FCP)

The time until the browser first renders any text or image from the page.

Total Blocking Time (TBT)

A lab metric: the sum of the time beyond 50 milliseconds in each long task after FCP. Useful as a lab proxy for INP, not a substitute for it. See JavaScript Weight in Casino Websites.

Long task

Any task on the main thread that runs longer than 50 milliseconds. While it runs, the page cannot respond to input.

Long animation frame (LoAF)

A rendering update delayed beyond 50 milliseconds. The Long Animation Frames API, shipped in Chrome 123, attributes such frames to the scripts that caused them. See Interaction to Next Paint (INP).

Main thread

The browser thread that runs most JavaScript, styles, layout and paint for a page. Work here competes with responses to user input.

Field data

Measurements from real visits on real devices and networks, such as CrUX or your own real-user monitoring. See Field vs Lab Data.

Lab data

Measurements from a controlled test, such as Lighthouse, run on one device profile and network setting. Good for debugging, not for judging what users experienced. See Field vs Lab Data.

Chrome UX Report (CrUX)

Google's public dataset of field measurements from opted-in Chrome users on eligible pages and origins. Chrome on iOS is not included. See Field vs Lab Data.

75th percentile

The value that 75% of page views meet or beat. Core Web Vitals use it so that a page passes only when most visits are good.

Lighthouse

Google's open source lab auditing tool. Its performance score weights five lab metrics and varies between runs.

PageSpeed Insights

A Google tool that shows CrUX field data for the previous 28 days alongside a Lighthouse lab run for the same URL.

Real-user monitoring (RUM)

Collecting performance data from your own visitors, for example with Google's open source web-vitals library. See Measurement Methodology.

Session window

In CLS, a group of layout shifts less than one second apart, lasting at most five seconds. CLS reports the window with the highest total.

Fetch priority

The fetchpriority attribute (high, low or auto) hints how urgently the browser should download a resource, such as the likely LCP image. See Image Optimization for Game Thumbnails.

Lazy loading

Deferring offscreen images and iframes until the reader scrolls near them, with loading="lazy". Never use it on the likely LCP image. See Image Optimization for Game Thumbnails.

Preconnect

A resource hint that opens a connection to another origin early. Browsers close unused connections after about 10 seconds, so keep it to critical origins.

Back/forward cache (bfcache)

A browser feature that restores a page instantly from memory on back or forward navigation. Unload handlers can make a page ineligible. See Caching and Delivery.

Code splitting

Breaking a JavaScript bundle into parts so that code needed later, such as a filter panel, loads only when it is used. See JavaScript Weight in Casino Websites.

content-visibility

A CSS property whose auto value lets the browser skip rendering offscreen sections until they approach the viewport. See Rendering Path Analysis.

Facade

A light placeholder, such as a static chat button, that loads the full third-party widget only when someone uses it. See The Cost of Third-Party Scripts.

Performance budget

A limit a team agrees on, such as total transfer size or JavaScript size per page, and checks before each release. See Setting Performance Budgets.

Transfer size

The number of bytes sent over the network for a resource after compression, as opposed to its size once decoded.

Throttling

Slowing the network or CPU in a test to imitate a slower device. Lighthouse's standard mobile setting uses 150 ms latency, 1.6 Mbps download and a 4x CPU slowdown.

Speculation rules

A Chromium API that lets a page ask the browser to prefetch or prerender likely next pages. Other browsers ignore the rules. See Caching and Delivery.

Render-blocking resource

A stylesheet or synchronous script that the browser must download and process before it can render the page.

Immutable caching

The Cache-Control immutable extension (RFC 8246) tells the browser that a response will not change while it is fresh, so it need not revalidate it. See Caching and Delivery.

WebRTC

A set of protocols for real-time audio, video and data between browsers, described in RFC 8825. Some live dealer streams use it. See Live Video and Web Performance.

Tag manager

A tool that loads marketing and analytics tags from one container. Each tag still adds requests and main-thread work. See The Cost of Third-Party Scripts.

Sources

1. web.dev, How the Core Web Vitals metrics thresholds were defined, last updated May 7, 2025.
2. web.dev, Interaction to Next Paint (INP), last updated September 2, 2025.
3. web.dev, Cumulative Layout Shift (CLS), last updated April 12, 2023.
4. Chrome for Developers, CrUX methodology, last updated June 20, 2024.
5. web.dev, Efficiently load third-party JavaScript, last updated August 14, 2019.
6. IETF, RFC 8246: HTTP Immutable Responses, September 2017.
7. IETF, RFC 8825: Overview: Real-Time Protocols for Browser-Based Applications, January 2021.