Core Web Vitals monitoring

Monitor page-experience signals without pretending lab data is field data.

Track synthetic LCP and CLS in a controlled browser, use TBT to investigate blocking work, and connect metric changes to the resources and scripts that changed.

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Synthetic LCP and CLSTBT and JS diagnosticsNo fabricated INP

Why it matters

Core Web Vitals are useful only when the methodology stays clear.

Lab and field measurements answer different questions. Controlled synthetic tests are excellent for repeatable debugging and regression detection, while field metrics describe what real visitors experience across actual devices and interactions.

SiteState keeps that distinction explicit. Synthetic browser runs track LCP and CLS and use Total Blocking Time as a diagnostic signal for main-thread work. SiteState does not invent Interaction to Next Paint from a lab run because INP requires real interaction data.

What SiteState monitors

Track page-experience changes with the evidence behind them

Largest Contentful Paint

Track synthetic LCP over time and inspect the LCP-related resources involved in the browser load.

Cumulative Layout Shift

Keep a repeatable synthetic CLS history so layout instability changes can be spotted over time.

Total Blocking Time

Use TBT as a lab diagnostic for main-thread blocking work without relabeling it as INP.

LCP-critical resources

Identify resources classified as important to the largest-contentful-paint path and prioritize investigation.

JavaScript main-thread impact

Attribute sampled CPU time to first-party and third-party scripts when JavaScript work becomes a likely bottleneck.

Regression history

Compare current synthetic behavior with recent baselines so a persistent deterioration is visible as a regression rather than a single red test.

Built for real workflows

Useful when performance work needs repeatable measurements

SEO and content teams

Track whether page-experience signals move after template, image, tag or content changes without confusing lab measurements with field rankings.

Developers

Use browser evidence, blocking resources and script attribution to investigate what changed behind a metric regression.

Agencies

Keep a consistent synthetic methodology across client sites and share operational status separately from private diagnostic detail.

State, not metrics

Transparent methodology is part of the product.

SiteState labels its browser measurements as lab data and keeps the test profile consistent so historical comparisons are meaningful. That makes the data useful for debugging and regression detection even before field integrations are added.

INP is intentionally not estimated from synthetic runs. When SiteState adds real-user or external field data, those measurements should remain a separate layer rather than being blended into synthetic results.

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FAQ

Questions people ask before choosing a monitoring tool

Can SiteState monitor Core Web Vitals?

SiteState currently monitors synthetic page-experience signals including LCP and CLS in controlled Chromium runs. It also tracks TBT as a lab diagnostic for blocking work.

Does SiteState measure INP in synthetic tests?

No. INP is an interaction metric based on real user interaction data. SiteState does not fabricate an INP value from a synthetic browser load.

Why can lab and field Core Web Vitals be different?

Lab tests use a controlled environment, while field data reflects real users, devices, networks and interactions. The two datasets are useful for different purposes and should not be treated as interchangeable.

Can SiteState explain why LCP gets worse?

SiteState combines LCP history with resource classification, page weight and JavaScript diagnostics so you can investigate likely contributors to a regression.

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