# Performance Metrics Reference Deep-dive reference for each Core Web Vital metric captured by the seo-intel system. ## The Five Metrics | Metric | Full Name | Unit | Good | Poor | What Matters | |--------|-----------|------|------|------|--------------| | **LCP** | Largest Contentful Paint | milliseconds | ≤2,500ms | ≥4,000ms | When main content appears | | **FCP** | First Contentful Paint | milliseconds | ≤1,800ms | ≥3,000ms | When ANY content appears | | **CLS** | Cumulative Layout Shift | unitless (0-1) | ≤0.1 | ≥0.25 | How much page jumps | | **TBT** | Total Blocking Time | milliseconds | ≤200ms | ≥600ms | JS blocking interactions | | **TTFB** | Time to First Byte | milliseconds | ≤800ms | ≥1,800ms | Server response speed | --- ## 1. LCP — Largest Contentful Paint ### Definition The time when the **largest visible element** (image, heading, paragraph block, video) appears on screen. ### Why It Matters Users need to see that the page is loading. LCP is the best metric for "when does the user perceive the page is starting to load?" ### Thresholds - **≤ 2.5 seconds:** Good — user feels the page is responding - **2.5 – 4.0 seconds:** Needs improvement - **≥ 4.0 seconds:** Poor — user thinks the page is slow/broken ### What Affects LCP 1. **Server response time (TTFB)** — If the server is slow, everything downstream is slow 2. **Large images/videos above the fold** — Unoptimized media delays LCP 3. **Render-blocking JavaScript** — `