> ## Documentation Index > Fetch the complete documentation index at: https://docs.vortexiq.ai/llms.txt > Use this file to discover all available pages before exploring further. # Lab Load Speed, Website Performance (PageSpeed + CrUX) > Lab Load Speed for Website Performance (PageSpeed + CrUX) stores. Tracked live in Vortex IQ Nerve Centre. How to read it, why it matters, and how to act on... **Metrics type:** [Supporting Metrics](/nerve-centre/overview#metrics-types-explained) • **Category:** [Website Performance](/nerve-centre/connectors#connectors-by-type) ## 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`](/nerve-centre/kpi-cards/website-performance/page-load-speed) (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`](/nerve-centre/kpi-cards/website-performance/performance-score-summary)). | | | | -------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | **What it counts** | The Largest Contentful Paint timestamp from a single Lighthouse audit run, in milliseconds from `navigationStart`. The LCP element is auto-detected as the largest above-the-fold render-area element (image, video poster, or text block). | | **Sample type** | **Lab data** (synthetic Lighthouse run). Field equivalent: [`crux_lcp_p75`](/nerve-centre/kpi-cards/website-performance/page-load-speed). | | **Why lab and field can differ** | Lab measures one synthetic load on emulated mid-tier mobile + slow 4G; field measures the real-user distribution. **Common gaps**: (a) lab cold-cache vs warm-cache real-user mix; (b) lab uses Google's PSI infrastructure (high CPU + good network); (c) real-user devices range from flagship phones (faster than lab) to entry-level Android (slower). The [`psi_field_vs_lab`](/nerve-centre/kpi-cards/website-performance/field-vs-lab) card surfaces the gap directly. | | **Lighthouse scoring** | Lab LCP feeds the Performance Score with these scoring breakpoints: ≤ 2,000ms = 100 score, 2,000-4,000ms = scaled, > 4,000ms = 0 score. **The 2,000ms target** is stricter than the field 2,500ms "good" threshold because lab conditions are deterministic and Lighthouse expects sites to ship with margin. | | **Device profile** | **Mobile by default** (Slow 4G + mid-tier CPU emulation). Desktop profile available via [`psi_mobile_desktop_lcp`](/nerve-centre/kpi-cards/website-performance/load-speed-mobile-vs-desktop). | | **Run-to-run variance** | Lab LCP fluctuates ±10-20 percent between runs due to timing variations in JS execution and network response. **A single audit's LCP is directional, not precise.** Use 7-day rolling for trend reads; compare deltas larger than 200ms as meaningful. | | **Why this card matters** | Engineering teams iterating on performance work need a deterministic measurement they can run repeatedly during development. Lab LCP fills that role: deploy a fix, run Lighthouse, see the impact within minutes. The field equivalent has a 28-day rolling lag that makes iteration impossible. | | **Currency** | n/a, this is a duration in milliseconds. | | **Time window** | `T/7D` (current measurement plus 7-day rolling average). | | **Alert trigger** | `> 2500ms` (matches field threshold for consistency). Sub-thresholds: amber 2,000-2,500ms, red > 2,500ms (lab band stricter than field by design). | | **Sentiment key** | `psi_lcp` | | **Roles** | owner, operations | ## 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. ## Worked example A UK-based BigCommerce fashion store homepage, lab measurement Wednesday 15 May 26. | Audit run | Lab LCP (mobile) | Lab LCP (desktop) | Field LCP (mobile p75) | Notes | | ---------------------------- | ---------------: | ----------------: | ---------------------: | ----------------------------------- | | Run 1 (current) | 4,420ms | 1,920ms | 4,820ms | Hero PNG carousel still unoptimised | | Run 2 (24h ago) | 4,580ms | 1,840ms | (28D rolling) | Within run-to-run variance | | Run 3 (48h ago) | 4,680ms | 2,020ms | | Within variance | | Run 4 (7 days ago, baseline) | 4,520ms | 1,960ms | | Within variance | | **7-day rolling average** | **4,550ms** | **1,935ms** | **4,820ms** | Lab and field agree directionally | 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`](/nerve-centre/kpi-cards/website-performance/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`](/nerve-centre/kpi-cards/website-performance/page-load-speed) 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: | Time horizon | Action | | -------------- | --------------------------------------------------- | | First 1 hour | Identify LCP element + dominant timeline portion. | | First 4 hours | Apply image format / preload / render-blocking fix. | | First 24 hours | Re-run Lighthouse; confirm lab LCP improvement. | | Day 7-28 | Field LCP rolling window absorbs. | ## Sibling cards merchants should reference together | Card | Why merchants reach for it | | ------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------- | | [`crux_lcp_p75`](/nerve-centre/kpi-cards/website-performance/page-load-speed) | Field LCP; the ranking-impact truth source. | | [`psi_field_vs_lab`](/nerve-centre/kpi-cards/website-performance/field-vs-lab) | Direct lab vs field gap analysis. | | [`psi_lab_tbt`](/nerve-centre/kpi-cards/website-performance/page-freeze-time) | Lab Total Blocking Time; pairs with lab LCP for the lab snapshot. | | [`psi_lab_cls`](/nerve-centre/kpi-cards/website-performance/lab-layout-stability) | Lab CLS. | | [`psi_speed_index`](/nerve-centre/kpi-cards/website-performance/speed-index) | Speed Index; alternative lab measure of perceived load speed. | | [`psi_performance_score`](/nerve-centre/kpi-cards/website-performance/performance-score) | Lab Performance Score; LCP is 25 percent of the weight. | | [`psi_perf_score_summary`](/nerve-centre/kpi-cards/website-performance/performance-score-summary) | Composite executive view. | | [`psi_image_optimisation`](/nerve-centre/kpi-cards/website-performance/image-optimisation) | Most common LCP fix lever. | | [`psi_render_blocking`](/nerve-centre/kpi-cards/website-performance/render-blocking-resources) | Render-blocking resources delay LCP indirectly. | | [`psi_slowest_lcp_urls`](/nerve-centre/kpi-cards/website-performance/slowest-lcp-urls) | Per-URL LCP ranking. | ## Reconciling against the vendor's own dashboard **Where to look:** * **[PageSpeed Insights](https://pagespeed.web.dev)**, "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:** | Reason | Direction | What to do | | --------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------ | -------------------------------------------------------------------------------------- | | **Run-to-run variance.** ±10-20 percent typical. | Either direction | Use 7-day rolling for trend reads. | | **Audit timing.** PSI runs on-demand at the moment of request; Vortex IQ runs scheduled. | Different snapshot times | Re-run PSI immediately after a Vortex IQ audit for direct comparison. | | **Local Lighthouse vs PSI.** Local DevTools Lighthouse runs on your CPU/network; PSI uses Google's infrastructure. Local often runs faster (better connection). | Local **lower** | Use the PSI web UI for direct comparison; local DevTools is for development iteration. | **Cross-connector reconciliation:** primarily internal (with [`crux_lcp_p75`](/nerve-centre/kpi-cards/website-performance/page-load-speed), [`psi_field_vs_lab`](/nerve-centre/kpi-cards/website-performance/field-vs-lab), [`psi_perf_score_summary`](/nerve-centre/kpi-cards/website-performance/performance-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. ## 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. *** ### 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](https://app.vortexiq.ai/login) or [book a demo](https://www.vortexiq.ai/contact-us) to see this metric running on your own data.