Lighthouse Metrics
Performance Score Weights
| Metric | Weight | Good | Needs Improvement | Poor |
|---|---|---|---|---|
| FCP First Contentful Paint | 10% | ≤ 1.8s | ≤ 3.0s | > 3.0s |
| LCP Largest Contentful Paint | 25% | ≤ 2.5s | ≤ 4.0s | > 4.0s |
| TBT Total Blocking Time | 30% | ≤ 200ms | ≤ 600ms | > 600ms |
| CLS Cumulative Layout Shift | 25% | ≤ 0.1 | ≤ 0.25 | > 0.25 |
| Speed Index | 10% | ≤ 3.4s | ≤ 5.8s | > 5.8s |
Metric Descriptions
| Metric | What it measures | Key optimizations |
|---|---|---|
| FCP | Time until first text or image is rendered | Eliminate render-blocking resources; preload critical fonts |
| LCP | Time until the largest visible element is rendered | Optimize hero images; use CDN; preload LCP resource |
| TBT | Sum of time the main thread is blocked (long tasks) | Split long JS tasks; reduce third-party scripts; code splitting |
| CLS | Amount of unexpected visual shifts | Set image dimensions; reserve ad space; avoid DOM insertions above fold |
| Speed Index | How quickly content is visually populated | Optimize critical rendering path; reduce CSS; defer offscreen images |
Score Ranges
| Score | Color | Category |
|---|---|---|
| 90–100 | Green | Good |
| 50–89 | Orange | Needs Improvement |
| 0–49 | Red | Poor |