Core Web Vitals are field metrics for loading, responsiveness and visual stability. Google’s current good thresholds are LCP at 2.5 seconds or less, INP at 200 milliseconds or less and CLS at 0.1 or less, evaluated at the 75th percentile of visits.
LCP measures main content loading
Largest Contentful Paint reports when the largest visible image or text block is rendered. Server response, image delivery, fonts, CSS and client-side rendering can all affect it.
INP measures interaction responsiveness
Interaction to Next Paint reflects the delay across user interactions. Heavy JavaScript, long tasks and expensive rendering can make a page feel unresponsive even after it looks loaded.
CLS measures visual stability
Cumulative Layout Shift captures unexpected movement. Images without dimensions, late banners, font changes and injected content are common causes.
Field and lab data answer different questions
Field data describes real visitors over time. Lab tests provide a controlled diagnostic snapshot. A good lab score cannot guarantee the 75th percentile field result.
Prioritise by real pages
Start with high-value templates and the actual LCP element, interaction or shift source. Avoid a long generic optimisation list without confirming the bottleneck.
- Landing pages
- Product or service templates
- Checkout or enquiry journeys
- Pages with poor field data
- Pages carrying heavy third-party scripts
Performance is shared ownership
Design, content, development, hosting and marketing tools all affect the result. Protect a performance budget when new scripts and content are added after launch.