core-web-vitals

Core Web Vitals

Measure the Core Web Vitals of any page using real browser performance APIs. No signup required.

Prerequisites

• Playwright MCP (comes with Claude Code)

Trigger

• "Check Core Web Vitals for https://..."
• "How's the LCP/CLS on my site?"
• "Web vitals audit mysite.com"

Workflow

1. Navigate to the URL using mcp__playwright__browser_navigate
2. Let the page settle (mcp__playwright__browser_wait_for ~3s, or wait for network idle)
3. Collect metrics with mcp__playwright__browser_evaluate:

() => new Promise((resolve) => {
  const out = {};
  // Navigation timing → TTFB, FCP
  const nav = performance.getEntriesByType('navigation')[0];
  if (nav) out.ttfb = Math.round(nav.responseStart);
  const fcp = performance.getEntriesByName('first-contentful-paint')[0];
  if (fcp) out.fcp = Math.round(fcp.startTime);

  // LCP
  try {
    new PerformanceObserver((list) => {
      const e = list.getEntries();
      out.lcp = Math.round(e[e.length - 1].startTime);
    }).observe({ type: 'largest-contentful-paint', buffered: true });
  } catch (e) {}

  // CLS (cumulative)
  let cls = 0;
  try {
    new PerformanceObserver((list) => {
      for (const entry of list.getEntries()) {
        if (!entry.hadRecentInput) cls += entry.value;
      }
      out.cls = Math.round(cls * 1000) / 1000;
    }).observe({ type: 'layout-shift', buffered: true });
  } catch (e) {}

  // Give observers a tick to flush
  setTimeout(() => resolve(out), 1500);
})

1. INP can't be measured passively — note it requires interaction. If you can click a primary button via mcp__playwright__browser_click, measure the event timing entry; otherwise report INP as "not measured (needs interaction)".

2. Grade each metric (Google thresholds):

Metric	Good	Needs work	Poor
LCP	≤ 2500ms	2500–4000ms	> 4000ms
CLS	≤ 0.1	0.1–0.25	> 0.25
INP	≤ 200ms	200–500ms	> 500ms
TTFB	≤ 800ms	800–1800ms	> 1800ms
FCP	≤ 1800ms	1800–3000ms	> 3000ms

1. Overall grade: A = all Good · B = one "Needs work" · C = two/three · D = any Poor · F = multiple Poor.

2. Output:

## Core Web Vitals Report: [URL]

**Grade: B** — one metric needs work

| Metric | Value   | Rating       |
|--------|---------|--------------|
| LCP    | 2.9s    | Needs work   |
| CLS    | 0.04    | Good         |
| INP    | (not measured — needs interaction) |
| TTFB   | 410ms   | Good         |
| FCP    | 1.6s    | Good         |

### What's hurting LCP
The largest element rendered at 2.9s — likely a hero image without
priority loading. Add `fetchpriority="high"` and preload it; serve in
AVIF/WebP; ensure it isn't behind render-blocking JS.

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