Web Performance Glossary

Definitions and explanations of Core Web Vitals, Lighthouse metrics, and web performance terminology.

Core Web Vitals

LCP, INP, and CLS measure loading, responsiveness, and visual stability in field data.

CLS

CLS measures visual stability. Learn what it is, thresholds, and how to prevent layout shifts.

INP

INP measures responsiveness. Learn what it is, thresholds, and how to improve interaction speed.

LCP

LCP measures loading performance. Learn what it is, thresholds, and how to improve your score.

Lighthouse Performance Metrics

Additional metrics reported by Lighthouse that contribute to your performance score.

Speed Index

Speed Index (SI) measures how fast visible content fills the viewport. Good: ≤3.4s mobile, ≤1.3s desktop. 10% of Lighthouse performance score. Thresholds, tools, fixes.

FCP

FCP measures when the first content renders. Learn what it is, thresholds, and how to improve it.

TBT

TBT measures main thread blocking during load. Learn what it is, thresholds, and how to reduce blocking time.

TTFB

TTFB helps explain loading delays. It does not directly contribute to the Lighthouse Performance score.

TTFB

Learn what time to first byte measures, the 800ms field guideline, and how to find delays in connections, redirects, and server work.

Deprecated Metrics

TTI (Deprecated)

TTI measured page interactivity but was deprecated in Lighthouse 10. Learn what replaced it.

Measure Your Site

Use Unlighthouse to audit all these metrics across your entire website in a single scan.

$npxunlighthouse--siteexample.com

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