core-web-vitals

Core Web Vitals

Measure LCP, CLS, TTFB, and FCP with real browser performance APIs and grade them against web.dev thresholds. No signup required.

Prerequisites

• Playwright MCP (ships with Claude Code)

Trigger

• "Check Core Web Vitals for https://..."
• "Measure LCP on my landing page"
• "Is my CLS under 0.1?"
• "Why does my page feel slow to load?"

Workflow

1. mcp__playwright__browser_navigate to the URL, then wait ~3s so late layout shifts and the final LCP candidate land.
2. mcp__playwright__browser_evaluate with this function (returns a Promise — Playwright awaits it):

() => new Promise((resolve) => {
  const out = { lcp: null, lcpElement: null, cls: 0, ttfb: null, serverWait: null, fcp: null };
  const nav = performance.getEntriesByType('navigation')[0];
  if (nav) {
    // web.dev TTFB: first response byte relative to navigation start (includes redirects, DNS, TLS)
    out.ttfb = Math.round(nav.responseStart - (nav.activationStart || 0));
    // Server think time only (responseStart - requestStart) — splits backend cost from connection cost
    out.serverWait = Math.round(nav.responseStart - nav.requestStart);
  }
  const fcp = performance.getEntriesByName('first-contentful-paint')[0];
  if (fcp) out.fcp = Math.round(fcp.startTime);
  try {
    new PerformanceObserver((list) => {
      const es = list.getEntries(), lastE = es[es.length - 1];
      out.lcp = Math.round(lastE.startTime);
      out.lcpElement = lastE.element ? lastE.element.outerHTML.slice(0, 120) : (lastE.url || '');
    }).observe({ type: 'largest-contentful-paint', buffered: true });
  } catch (e) {}
  // CLS = worst session window (entry gap < 1s, window span < 5s); input-caused shifts excluded
  let max = 0, session = 0, first = 0, last = 0;
  try {
    new PerformanceObserver((list) => {
      for (const e of list.getEntries()) {
        if (e.hadRecentInput) continue;
        if (session && e.startTime - last < 1000 && e.startTime - first < 5000) session += e.value;
        else { session = e.value; first = e.startTime; }
        last = e.startTime;
        if (session > max) max = session;
      }
      out.cls = Math.round(max * 1000) / 1000;
    }).observe({ type: 'layout-shift', buffered: true });
  } catch (e) {}
  setTimeout(() => resolve(out), 2000);
})

1. INP is not measured here. INP needs real user interactions across the visit; a passive load has none. Report it as "not measured (needs interaction)" — never fake it. Optionally click one primary control with mcp__playwright__browser_click and read performance.getEntriesByType('event') for a single-interaction latency sample, clearly labeled as such.
2. Grade each metric against web.dev thresholds (75th-percentile targets):

Metric	Good	Needs improvement	Poor
LCP	≤ 2500 ms	2500–4000 ms	> 4000 ms
CLS	≤ 0.1	0.1–0.25	> 0.25
TTFB	≤ 800 ms	800–1800 ms	> 1800 ms
FCP	≤ 1800 ms	1800–3000 ms	> 3000 ms

1. Overall grade: A = all Good · B = one Needs improvement · C = two+ Needs improvement · D = one Poor · F = multiple Poor. Note this is one lab run on one machine — field data (CrUX) may differ.

Report

## Core Web Vitals Report: [URL]

**Grade: B** — one metric needs improvement (web.dev thresholds, single lab run)

| Metric | Value | Rating |
|--------|-------|--------|
| LCP  | 2.9 s  | Needs improvement |
| CLS  | 0.04   | Good |
| TTFB | 410 ms | Good (server wait: 180 ms) |
| FCP  | 1.6 s  | Good |
| INP  | not measured — needs real interaction |

### What's hurting LCP
LCP element: [outerHTML snippet]. Rendered at 2.9 s — check for
render-blocking JS/CSS, missing `fetchpriority="high"`, or an
unoptimized hero image (serve AVIF/WebP, preload it).

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