The CrUX Field Data Probe is the audit you reach for when you already suspect a problem in this dimension and need a fast, copy-paste-able fix list. It reuses the same chrome as every other jwatte.com tool — deep-links from the mega analyzers, AI-prompt export, CSV/PDF/HTML download — but the checks it runs are narrow and specific.
Fetches real-user Chrome UX Report p75 field data via the PageSpeed Insights API (no API key needed for low volume). Shows actual user experience metrics Google ranks on, not lab simulation.
What it actually checks
This is a partial extract of the audit's real findings — the same strings the tool prints when a check trips. Use it as a quick sanity check before you run the audit live:
Why this dimension matters
Core Web Vitals (LCP ≤ 2.5s, INP ≤ 200ms, CLS ≤ 0.1) are a Google ranking factor. They also gate how often a page can appear in AI Overviews — slow pages get deprioritized in the answer-generation step after the retrieval step. Field data (CrUX, 28-day rolling real-user) beats lab data (Lighthouse) for this reason: Google indexes field metrics, not synthetic ones.
Common failure patterns
- LCP hero image with
loading="lazy"— lazy-loading the LCP element defers it beyond the above-the-fold paint. The rule is: LCP image = no lazy, addfetchpriority="high", preload it in<link rel="preload" as="image">with the rightimagesrcsetfor responsive variants. - CLS from late-loading web fonts —
font-display: swapwithoutsize-adjust/ascent-overridefallback produces a layout shift at font load. Usefont-display: optionalon non-critical fonts, or match metrics precisely with a fallback stack. - INP from long tasks on interaction — a single
main-threadtask over 50ms blocks next-paint. The fix is breaking up handlers withrequestIdleCallback,scheduler.yield(), or post-message loops. Third-party analytics scripts are the most common culprit;defer+Partytown/ WebWorker offload helps. - TTFB regressions from cold serverless — scheduled functions with per-region cold starts add 300–1500ms to every geographically-distant visitor. Edge functions (Cloudflare Workers, Vercel Edge, Netlify Edge Functions) cut this to ~50ms.
How to fix it at the source
Treat CrUX field data as the authoritative measure; Lighthouse lab data is a proxy. Instrument your own RUM (the web-vitals library emits LCP/CLS/INP/TTFB/FCP to any endpoint) for percentile breakdowns beyond CrUX's 28-day window. For images: preload the LCP, lazy-load everything below the fold, always provide intrinsic width/height. For scripts: defer or async or Web Worker — never sync at the top of <head>.
When to run the audit
- After a major site change — redesign, CMS migration, DNS change, hosting platform swap.
- Quarterly as part of routine technical hygiene; the checks are cheap to run repeatedly.
- Before an investor / client review, a PCI scan, a SOC 2 audit, or an accessibility-compliance review.
- When a downstream metric drops (rankings, conversion, AI citations) and you need to rule out this dimension as the cause.
Reading the output
Every finding is severity-classified. The playbook is the same across tools:
- Critical / red: same-week fixes. These block the primary signal and cascade into downstream dimensions.
- Warning / amber: same-month fixes. Drag the score, usually don't block.
- Info / blue: context-only. Often what a PR reviewer would flag but that doesn't block merge.
- Pass / green: confirmation — keep the control in place.
Every audit also emits an "AI fix prompt" — paste into ChatGPT / Claude / Gemini for exact copy-paste code patches tied to your stack.
Related tools
- Mega Analyzer — One URL, every SEO/schema/E-E-A-T/voice/mobile/perf audit in one pass..
- Image LCP Candidate Audit — Identifies the likely LCP image, flags loading="lazy" conflict, missing width/height, no srcset, legacy formats..
- Font Loading Strategy Audit — Audits @font-face font-display, preload hints, woff2 coverage, Google Fonts vs self-hosted, FOUT/FOIT pattern risks, variable fonts..
- Critical CSS Inlining Audit — Counts render-blocking stylesheets, inlined