Photo by Jakub Żerdzicki on Unsplash
Core Web Vitals discussions usually default to SEO ranking. The more useful framing is conversion, and the case studies are public. Renault tied a 1-second improvement in Largest Contentful Paint to a 13% conversion lift and a 14-percentage-point bounce-rate reduction, measured across roughly 10 million page visits (web.dev Renault case study, 2021). Vodafone A/B-tested a 31% LCP improvement with no UX changes and saw an 8% sales increase plus a 15% lead-to-visit uplift (web.dev Vodafone, 2023).
The pattern repeats across very different verticals. Rakuten 24, an ecommerce property, A/B-tested LCP optimisations and reported a 33.13% conversion-rate increase plus a 53.37% lift in revenue per visitor (web.dev Rakuten, 2023). Tokopedia's optimisation work, which combined 88% script-size reduction with image optimisation and a Svelte rebuild of the homepage, lifted conversion 8% and click-through 35% (web.dev Tokopedia, 2021).
The recent additions to the Vital set show this is not an LCP-only story. Yahoo! Japan News reduced Cumulative Layout Shift by 0.2 and saw a 15.1% increase in page views per session, a 13.3% increase in session duration, and a 1.72-percentage-point bounce reduction (web.dev Yahoo! Japan News, 2021). Fotocasa, after Interaction to Next Paint became a Core Web Vital in March 2024 (web.dev INP-CWV transition), removed unnecessary rerenders and optimised third-party script execution, growing contact and phone lead conversions by 27% (web.dev Fotocasa, 2024).
A caveat the practitioner literature underuses: Renault's data shows the bounce-rate gain from LCP improvements clusters under the 1.6-second mark (14 percentage points below 1.6s, only 5 above), suggesting performance has thresholds rather than linear returns. Optimise to the threshold, not just below the technical pass mark, if you want the conversion curve to actually move.