# Web performance glossary: LCP, CLS, INP and more

> Plain-English definitions of LCP, CLS, INP, TBT, FCP, TTFB, Speed Index, CrUX, Lighthouse scores, Agentic Browsing and Core Web Vitals.

- **[Agentic Browsing](https://airopages.com/glossary/agentic-browsing.md):** A Lighthouse category (13.3+, beta) that checks how usable a page is for AI agents. Shown as checks passed, not as a 0 to 100 score.
- **[Chrome UX Report (CrUX)](https://airopages.com/glossary/crux.md):** CrUX is a public dataset of real-user performance from Chrome, reported at the 75th percentile over a rolling 28-day window.
- **[Core Web Vitals](https://airopages.com/glossary/core-web-vitals.md):** Core Web Vitals are three user-experience metrics, LCP, INP and CLS, assessed on real-user data at the 75th percentile.
- **[Cumulative Layout Shift (CLS)](https://airopages.com/glossary/cls.md):** CLS measures how much visible content moves around unexpectedly while a page loads and is used. A good CLS is 0.1 or less.
- **[First Contentful Paint (FCP)](https://airopages.com/glossary/fcp.md):** FCP measures when the browser first renders any text, image or non-white canvas from the page. A good FCP is 1.8 seconds or less.
- **[Interaction to Next Paint (INP)](https://airopages.com/glossary/inp.md):** INP measures how quickly a page responds visually to clicks, taps and key presses across a visit. A good INP is 200 milliseconds or less.
- **[Largest Contentful Paint (LCP)](https://airopages.com/glossary/lcp.md):** LCP measures how long it takes for the largest image or text block in the viewport to render. A good LCP is 2.5 seconds or less.
- **[Lighthouse score](https://airopages.com/glossary/lighthouse-score.md):** A 0 to 100 rating for Performance, Accessibility, Best Practices and SEO. Performance is a weighted average of five lab metrics; 90+ is good.
- **[Speed Index (SI)](https://airopages.com/glossary/speed-index.md):** Speed Index is a Lighthouse lab metric that shows how quickly the visible content of a page fills in during load. Lower is better.
- **[Time to First Byte (TTFB)](https://airopages.com/glossary/ttfb.md):** TTFB measures the time from the request until the first byte of the response arrives. A good TTFB is 0.8 seconds or less.
- **[Total Blocking Time (TBT)](https://airopages.com/glossary/tbt.md):** TBT is a Lighthouse lab metric that adds up how long the main thread was blocked by long tasks after First Contentful Paint.