What Are Core Web Vitals?
Core Web Vitals are a set of performance metrics defined by Google that measure real-world user experience when visiting a website. Since 2021, Google has used Core Web Vitals as ranking factors in its search algorithm.
The 5 Key Metrics
1. LCP — Largest Contentful Paint
Measures: How long it takes to render the largest visible element on screen (usually the hero image or a large text block).
| Score | Value |
|---|---|
| ✅ Good | ≤ 2.5 seconds |
| ⚠️ Needs Improvement | 2.5–4.0 seconds |
| ❌ Poor | > 4.0 seconds |
How to improve: Optimize hero images (use WebP, compress), use a CDN, preload critical images.
2. FCP — First Contentful Paint
Measures: Time from navigation until the browser renders the first piece of content (text, image, canvas).
| Score | Value |
|---|---|
| ✅ Good | ≤ 1.8 seconds |
| ⚠️ Needs Improvement | 1.8–3.0 seconds |
| ❌ Poor | > 3.0 seconds |
How to improve: Optimize critical CSS, reduce render-blocking resources.
3. CLS — Cumulative Layout Shift
Measures: Total amount of unexpected layout shifts during the page's lifetime — e.g., buttons jumping around as images finish loading.
| Score | Value |
|---|---|
| ✅ Good | ≤ 0.1 |
| ⚠️ Needs Improvement | 0.1–0.25 |
| ❌ Poor | > 0.25 |
How to improve: Set fixed width and height on images; avoid injecting dynamic content above existing content.
4. TBT — Total Blocking Time
Measures: Total time the main thread is blocked long enough to prevent user input responses (between FCP and Time to Interactive).
| Score | Value |
|---|---|
| ✅ Good | ≤ 200ms |
| ⚠️ Needs Improvement | 200–600ms |
| ❌ Poor | > 600ms |
How to improve: Split large JavaScript bundles, remove unused scripts, use Web Workers.
5. TTFB — Time to First Byte
Measures: Time from when the browser sends a request to when it receives the first byte from the server.
| Score | Value |
|---|---|
| ✅ Good | ≤ 600ms |
| ⚠️ Needs Improvement | 600ms–1.8s |
| ❌ Poor | > 1.8s |
How to improve: Upgrade hosting, use servers close to users (CDN), enable caching.
How to Check Your Core Web Vitals
Use WebCheck to check all 5 metrics at once at webcheck.shunify.com — results appear in seconds, with specific improvement tips for each metric.