Web Performance Glossary
Core Web Vitals & Lighthouse Metrics
Plain-language definitions for every metric that affects your Lighthouse score and Google rankings. Each page includes thresholds, common causes, and actionable fixes.
LCP — Largest Contentful Paint
How fast the largest visible element loads
FCP — First Contentful Paint
When the first pixel of content appears
CLS — Cumulative Layout Shift
Visual stability during page load
TBT — Total Blocking Time
Main-thread blocking from JavaScript
INP — Interaction to Next Paint
Responsiveness to user interactions
TTFB — Time to First Byte
Server response latency before content
Speed Index
How quickly visible content populates
Lighthouse Score
How the 0–100 performance score is calculated
Core Web Vitals
Google's three ranking metrics: LCP, INP, and CLS