Mobile, primarily — Google has been mobile-first indexing since 2023, and mobile field data is the basis for the Core Web Vitals ranking signal. Desktop CWV is reported in Search Console for diagnostic purposes but isn't the primary ranking input.
Google's mobile-first indexing transition completed in 2023. Since then, the mobile version of your site is the canonical version Google uses for indexing and ranking.
**For Core Web Vitals specifically:**
- **Mobile field data drives the CWV ranking signal.** When Google evaluates whether your page passes CWV for ranking purposes, they use the mobile field data from CrUX. - **Desktop field data is reported separately in Search Console** for diagnostic purposes. Google looks at it but it's not the primary ranking input. - **If your site has more than 50% desktop traffic** (rare in 2026 — most sites are 60%+ mobile), desktop CWV may carry more weight in your specific ranking signals.
**Practical implications:**
**1. Optimize for mobile first.** Test on real mid-tier Android devices, not high-end iPhones or laptops. The performance gap between flagship and budget devices is significant — and Google's CWV data weights toward the slower end via the 75th percentile.
**2. Don't trust desktop Lighthouse scores.** A 95+ Lighthouse score on desktop tells you nothing about your mobile field performance. Always run mobile Lighthouse + check mobile field data.
**3. Mobile-specific optimizations matter more than desktop:**
- Touch-friendly tap targets (44×44px minimum) - Responsive images that don't ship 4000px-wide files to phones - Async-loading non-critical JavaScript that would block interactions on slow mobile networks - Avoiding hover-only interaction patterns - Limiting third-party scripts (each one costs more on mobile than desktop)
**4. The 4G assumption:** Lighthouse's "Mobile" preset simulates 4G with a 4× CPU slowdown. This approximates a mid-2018 mid-range Android on 4G. Real Canadian mobile users span:
- ~5% on 5G with flagship devices (effectively desktop performance) - ~50% on 4G with modern mid-range devices (close to Lighthouse simulation) - ~30% on 4G with 2–4 year old devices (slower than Lighthouse simulation) - ~15% on flaky/edge 4G or 3G (significantly slower)
If you only test on the first group, you optimize for 5% of your audience.
**5. Consider device diversity in your QA:**
A practical setup for any team serious about CWV:
- One newer iPhone (15 or 16) — represents the fast user - One mid-tier Android (Samsung A-series, 2–3 years old) — represents the median user - One older Android (Samsung A-series, 4+ years old) — represents the slow user - Test on flaky WiFi and on cellular hotspot with throttling
**Where desktop CWV matters more:**
B2B sites with predominantly desktop traffic (enterprise software, professional services targeting office workers) have desktop-skewed user distributions. Google's CWV data still leans mobile-first, but for sites where 80%+ of traffic is desktop, your competitive comparison is increasingly happening among other desktop-heavy sites — so desktop performance becomes your meaningful baseline regardless of Google's general ranking model.
**Search Console Core Web Vitals report:**
The report shows separate Mobile and Desktop tabs. Both display:
- Pages Good / Needs Improvement / Poor for each metric (LCP, INP, CLS) - Trends over time - Examples of URLs failing each threshold
Fix mobile issues first; then desktop. Most fixes apply to both, so working mobile-first usually improves desktop along the way.
- **What is INP and how do I fix poor INP scores?** — Interaction to Next Paint — measures how quickly your page responds to user input. Should be under 200ms (good) or under 500ms (acceptable). Replaced FID in March 2024. Most pages with poor INP have heavy JavaScript event handlers or excessive third-party scripts blocking the main thread. - **How do I fix a poor Largest Contentful Paint (LCP) score?** — LCP should be under 2.5 seconds on mobile. Five fixes that work for 90% of sites: (1) optimize and preload your hero image, (2) eliminate render-blocking resources above the fold, (3) use a CDN, (4) enable HTTP/2 or HTTP/3, (5) reduce server response time (TTFB) under 600ms. - **What's a good Core Web Vitals score in 2026?** — All three metrics in the 'Good' threshold (LCP <2.5s, INP <200ms, CLS <0.1) at the 75th percentile of mobile users over the trailing 28 days. About 40% of websites achieve this in 2026 — passing all three is a meaningful competitive edge. - **Lab vs field data — which one does Google actually use?** — Field data (real user measurements) is what Google uses for ranking. Lab data (synthetic Lighthouse runs) is for debugging only. A site can have perfect Lighthouse scores and still fail Core Web Vitals if real users experience poor performance.