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How to check Core Web Vitals for hundreds of URLs at once (API + code)

#webdev #webperf #seo #javascript

Google's PageSpeed Insights is great for auditing one page. But the moment
you want to track Core Web Vitals across a whole site — or every client site
you manage, or every page after a deploy — clicking through a web UI (or fighting
per-key rate limits) stops scaling.

Here's how to measure LCP, CLS, FCP and TTFB in bulk, in real Chromium, with
a plain API call and no rate-limit juggling.

What we're measuring

The metrics that actually affect Google ranking and user experience:

  • LCP (Largest Contentful Paint) — when the main content appears. Target < 2.5s.
  • CLS (Cumulative Layout Shift) — how much the layout jumps. Target < 0.1.
  • FCP (First Contentful Paint) — first pixel of content.
  • TTFB (Time To First Byte) — server responsiveness.

Plus the why: total page weight, request count, third-party load, DOM size —
the things you actually change to fix a bad score.

The bulk approach

Website Performance Audit
loads each URL in a real browser, measures Core Web Vitals via the standard
PerformanceObserver APIs (lab data, comparable to Lighthouse), and returns a
0–100 score plus a prioritized list of opportunities.

curl -X POST "https://api.apify.com/v2/acts/runlayer~website-performance-audit/run-sync-get-dataset-items?token=YOUR_APIFY_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{ "urls": ["https://example.com", "https://example.com/pricing", "https://example.com/blog"], "device": "mobile" }'
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Each URL comes back like this:

{
  "url": "https://example.com/pricing",
  "score": 72,
  "lcpMs": 2380, "cls": 0.04, "fcpMs": 1290, "ttfbMs": 410,
  "totalKb": 2648, "requests": 84, "thirdPartyRequests": 37,
  "opportunities": [
    { "severity": "warning", "title": "Render-blocking resources", "detail": "5 synchronous scripts/stylesheets in <head>…" },
    { "severity": "warning", "title": "Heavy JavaScript", "detail": "Scripts total 1180 KB…" }
  ]
}
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Make it a performance budget in CI

The useful part isn't a one-time number — it's catching regressions. Fail your
build if a key page's score drops or LCP crosses a threshold:

const urls = ["https://example.com", "https://example.com/pricing"];

const results = await fetch(
  "https://api.apify.com/v2/acts/runlayer~website-performance-audit/run-sync-get-dataset-items?token=" + process.env.APIFY_TOKEN,
  {
    method: "POST",
    headers: { "Content-Type": "application/json" },
    body: JSON.stringify({ urls, device: "mobile" }),
  }
).then((r) => r.json());

const budget = { score: 70, lcpMs: 2500, cls: 0.1 };
const failures = results.filter(
  (r) => r.score < budget.score || r.lcpMs > budget.lcpMs || r.cls > budget.cls
);

if (failures.length) {
  console.error("Performance budget exceeded:");
  for (const f of failures) console.error(`  ${f.url} — score ${f.score}, LCP ${f.lcpMs}ms, CLS ${f.cls}`);
  process.exit(1);
}
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Drop that in a GitHub Action after deploy and you get a hard gate on performance
regressions — across as many pages as you want, in one call.

Auditing a whole site on a schedule

Feed it your sitemap URLs (or pair it with a crawler) and run it on an Apify
Schedule weekly. Sort the resulting dataset by score ascending and you've
got a prioritized worklist: worst pages first, each with the specific
opportunities to fix.

SEO, not just speed?

Core Web Vitals are one ranking input. If you also want on-page SEO — titles,
meta descriptions, headings, canonical tags, structured data, broken links —
the companion SEO Audit actor
returns all of that plus Core Web Vitals in one record, each issue paired with
a concrete fix.

Cost

Pay per page audited, no subscription, no per-key rate limits, failures free.
Auditing a 100-page site is pocket change and runs in a couple of minutes — and
Apify's free credits cover plenty of testing.

Built this because bulk Core Web Vitals shouldn't require a spreadsheet of
PageSpeed tabs. Feedback and feature requests (INP? filmstrips?) via the actor's
issues tab. The full utility suite is at
apify.com/runlayer.

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