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Metrics type: Supporting Metrics • Category: Website Performance

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At a glance

Lab-measured Largest Contentful Paint from Lighthouse’s synthetic page-load. Reflects what LCP would be in standardised conditions (slow 4G, mid-tier mobile CPU emulation). The deterministic engineering metric that pairs with the field-data crux_lcp_p75 (real users): lab gives you a number you can run repeatedly during development to confirm fixes; field gives you the ranking-impact truth source. Lab LCP is what feeds 25 percent of the lab Performance Score (psi_perf_score_summary).

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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 homepage, lab measurement Wednesday 15 May 26. What the lab measurement is telling us:
  1. Lab LCP at 4,550ms (7-day rolling) sits in the “poor” band (above 4,000ms), and the field measurement at 4,820ms confirms real users experience similar slowness. Lab and field agree directionally, the gap of ~270ms between them is normal and reflects: cold-cache lab vs warm-cache real-user mix, lab’s mid-tier emulation vs real-user device distribution, and lab’s deterministic conditions vs real-user network variance.
  2. Run-to-run variance is visible in the daily measurements (4,420 → 4,580 → 4,680 → 4,520ms over 4 runs). The ±150ms range is typical Lighthouse variance; don’t react to single-run movements under 200ms. The 7-day rolling smooths to a stable 4,550ms baseline.
  3. Desktop at 1,935ms is in the “good” band (under 2,000ms target). Desktop runs faster because: (a) no network throttling (vs mobile slow 4G), (b) no CPU throttling (vs mid-tier mobile emulation), (c) typically larger viewport selecting different LCP elements. Desktop performance is not the priority; mobile is what Google ranks on.
  4. The gap between lab desktop (1,935ms) and lab mobile (4,550ms) is structural. Same site, same code, same audit infrastructure, only the throttling profile differs. A 2.5x mobile penalty is typical for unoptimised hero images: the slow 4G connection takes 2.5x longer to download the 2.8MB hero PNG, and that download time directly drives LCP.
  5. The fix priority: lift mobile lab LCP from 4,550ms to under 2,500ms via image optimisation (covered in psi_image_optimisation). Expected post-fix lab LCP: 1,800-2,200ms mobile, 1,200-1,400ms desktop. Field LCP follows over the 28-day rolling window.
  6. Engineering iteration cadence: deploy a fix, run Lighthouse manually (gets a fresh lab measurement in 60 seconds), confirm the lab LCP dropped as expected. Repeat for next fix. Lab is the right tool for this loop; waiting 7+ days for field-data confirmation makes iteration impossible.
The diagnostic flow when this card flags amber/red:
  1. Confirm with field data. crux_lcp_p75 and lab should agree directionally; large persistent gaps suggest real-user-condition-specific issues.
  2. Decompose the LCP timeline. Lighthouse’s audit JSON breaks LCP into TTFB + Resource load delay + Resource load duration + Element render delay. The dominant portion suggests the fix.
  3. Identify the LCP element. Lighthouse names the element (hero image, hero video, h1 text); the fix-pattern follows.
  4. Apply the fix; re-run lab Lighthouse to confirm. Iterate until lab LCP is under target.
  5. Wait 28 days for field validation. Field LCP starts shifting at 7-day rolling, fully reflects at 28 days.
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:
  • PageSpeed Insights, “Performance” panel shows lab LCP for the audited URL.
  • Chrome DevTools → Lighthouse panel, runs the same audit locally; lab LCP value matches PSI within run-to-run variance.
  • Lighthouse CI, runs in your build pipeline; lab LCP is the metric that fails builds when set as a budget.
Why the Vortex IQ lab LCP may differ from a fresh PSI run: Cross-connector reconciliation: primarily internal (with crux_lcp_p75, psi_field_vs_lab, psi_perf_score_summary). Quick rule for support tickets: if a merchant says “my local Lighthouse says LCP is 2.1s but your card shows 4.5s”, the most common cause is the merchant running Lighthouse on a fast connection without proper throttling. Confirm the local Lighthouse is set to “Mobile” device with “Slow 4G” throttling for direct comparison.

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Known limitations / merchant FAQs

Should I optimise to lab LCP or field LCP target? Lab for engineering iteration; field for ranking decisions. The lab target is stricter (2,000ms vs 2,500ms field) because lab is deterministic, Lighthouse expects sites to ship with margin so real-user variability still keeps you under the field threshold. Brands hitting 2,000ms lab LCP almost always have field p75 LCP under 2,500ms. My lab LCP improved but my field LCP didn’t move. What’s wrong? Three possibilities. (1) The 28-day field rolling window hasn’t caught up yet, wait 14-28 days post-fix. (2) The fix worked in synthetic conditions but not real-user conditions, common if the fix relies on warm cache that real users don’t have. (3) The fix worked on the audited URL but real-user traffic concentrates on different URLs, verify your audit URL list matches actual traffic distribution. My lab LCP fluctuates 600ms between runs. Is something broken? Not necessarily. Lighthouse run-to-run variance is typically ±10-20 percent for LCP. A 600ms range on a 4,000ms baseline (15 percent) is at the upper end of normal. Use 7-day rolling for trend reads; single-run movements under 200ms are noise. Why does desktop lab LCP run so much faster than mobile? Throttling. Lighthouse mobile uses “Slow 4G” (1.6 Mbps down) plus mid-tier mobile CPU emulation (4x slowdown). Desktop uses unthrottled connection and unthrottled CPU. The 2-3x mobile penalty is structural, not a sign your site is broken. Can I make Lighthouse run on my own connection? Locally, yes. Chrome DevTools Lighthouse defaults to throttled emulation but offers a “Use desktop performance throttling” option. However: local runs are for engineering iteration; production monitoring should use PSI/Vortex IQ which apply the canonical throttling profile. My lab LCP is 1,800ms. Am I done optimising? For LCP, mostly yes. 1,800ms is below the 2,000ms 100-score threshold. Marginal improvements below 1,800ms rarely translate to meaningful field improvement; effort shifts to other metrics (TBT for INP, CLS for stability) at this point.

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Tracked live in Vortex IQ Nerve Centre

Lab Load Speed 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.