Core Web Vitals Vocabulary

Vocabulary Reference

LCP — Largest Contentful Paint

Measures how long until the largest visible content element (image, heading, video poster) is rendered. Good: ≤ 2.5 s. "Our LCP is 2.1s" means it passes the good threshold.

FID — First Input Delay (deprecated)

The delay between a user's first interaction (click, tap) and the browser's response. Replaced by INP as an official Core Web Vital in March 2024.

INP — Interaction to Next Paint

Measures responsiveness across all interactions during the page lifespan, not just the first. Good: ≤ 200 ms. Replaced FID as a Core Web Vital.

CLS — Cumulative Layout Shift

Measures visual stability: the total score of unexpected layout shifts during page load. Good: ≤ 0.1. Common causes: images without dimensions, late-loading ads, injected banners.

PageSpeed Insights

Google's tool (pagespeed.web.dev) that runs Lighthouse in the lab and pulls real-user CrUX field data to report Core Web Vitals scores with "Good / Needs Improvement / Poor" thresholds.

TTFB — Time to First Byte

Server responsiveness metric: time from the navigation request until the first byte of the response arrives. Not a Core Web Vital but affects LCP. Good: ≤ 800 ms.