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Metrics type: Supporting Metrics • Category: Website Performance

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At a glance

First Contentful Paint p75 over time, daily time-series of mobile FCP from CrUX. Pairs with crux_fcp_p75 snapshot. The earliest paint signal trend, useful for distinguishing TTFB-driven regressions (FCP and TTFB rise together) from render-blocking-driven regressions (FCP rises alone) from LCP-element-driven regressions (LCP rises but FCP doesn’t). FCP is no longer a CWV (replaced by LCP in March 2020) but remains useful as a precursor metric.

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Calculation

Calculated automatically from your Website Performance (PageSpeed + CrUX) data. See the At a glance summary above for what the metric tracks and the worked example below for a typical reading.

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Worked example

A UK-based BigCommerce fashion store, mobile FCP + TTFB + LCP trio over 6 months ending Wednesday 15 May 26. What the three-trend trio is telling us:
  1. The trio of TTFB + FCP + LCP shows the regression staging clearly. Different metrics rose at different times, identifying different cause types:
    • Jan 26 FCP rise without TTFB rise = render-blocking resources added (Klaviyo popup CSS as external stylesheet)
    • Feb 26 FCP + LCP rise = render-blocking persisted + new heavy hero (the carousel deploy)
    • Mar 26 TTFB rise = server / cache regression (CDN cache hit rate dropped from 70% to 45%)
    • Apr-May 26 LCP-only rise = hero campaign images uploaded without optimisation
  2. Reading the trio diagnoses regression type without manual investigation. This is the value of the FCP trend, it’s the differential between TTFB and LCP, surfacing render-blocking issues that otherwise look like generic “everything got slower.”
  3. The recovery sequence follows the same logic, in reverse:
    • Address TTFB first (lift cache hit rate back to 80%+), pulls FCP and LCP down by ~400ms each just from TTFB improvement.
    • Address render-blocking next (defer Klaviyo popup CSS), pulls FCP and LCP down further.
    • Address LCP-element-specific issues (image format conversion, preload), pulls only LCP down.
  4. The trio also catches the inverse pattern: when only LCP rises and FCP stays stable, the regression is image-element-specific. No need to investigate render-blocking or TTFB; the diagnosis is unambiguous.
  5. Alert configuration recommendation: alert on FCP regressions specifically because they signal render-blocking issues that LCP alone wouldn’t isolate. A +400ms FCP delta over 7 days alert catches render-blocking regressions early.
The diagnostic flow:
  1. Check FCP trend alongside TTFB and LCP trends.
  2. Pattern-match the trio: which metrics rose, in what order? The pattern identifies the regression class.
  3. Apply the right fix per identified class.
Rapid-response playbook:

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Sibling cards merchants should reference together

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Reconciling against the vendor’s own dashboard

Where to look:
  • CrUX BigQuery, historical FCP data.
  • PageSpeed Insights, current FCP shown in field section.
  • Web Vitals Chrome extension, live FCP measurement.
Why the Vortex IQ FCP trend may differ from external views: Cross-connector reconciliation: primarily internal (with crux_fcp_p75, crux_lcp_trend, crux_ttfb_trend). Quick rule for support tickets: if a merchant says “my FCP regressed but TTFB is fine”, render-blocking resources were added. Check theme version, third-party CSS additions, font-loading configuration changes.

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Known limitations / merchant FAQs

Should I monitor FCP trend as a separate alert from LCP? Yes. FCP-only regressions (FCP up while LCP stable) catch render-blocking issues that LCP alone might miss because LCP is dominated by image-element timing. A +400ms FCP delta alert catches this class of regression early. Why is FCP no longer a CWV but still tracked? Google replaced FCP with LCP as the official CWV in March 2020 because FCP fires too easily (any tiny element render triggers it) without reflecting user perception. FCP remains useful diagnostically as a precursor to LCP and as a render-blocking detector. My TTFB and FCP trended up together. What does that suggest? Server / cache regression. TTFB is upstream of FCP; if both rise together, the issue is server-side (slow origin, cache miss rate increase, network configuration change). Investigate CDN cache hit rate first. Can I have FCP improve while LCP regresses? Yes, when render-blocking is reduced (FCP improves) but the LCP element is replaced with a heavier image (LCP regresses). The trio diagnostic catches this; the snapshot view alone wouldn’t.

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Tracked live in Vortex IQ Nerve Centre

First Content Over Time is one of hundreds of KPI pulses Vortex IQ tracks across Website Performance (PageSpeed + CrUX) and 70+ other ecommerce connectors. Nerve Centre runs the detection layer; Vortex Mind investigates the cause when something moves; Ask Viq lets you interrogate any number in plain English. Start for free or book a demo to see this metric running on your own data.