Core Web Vitals: how site speed relates to conversion

Core Web Vitals is a set of Google metrics that measure not "site speed" in a general sense but three specific user experiences: how quickly the main content appears, how responsive the interface is, and whether the page jumps around while loading.

The three metrics

• LCP (Largest Contentful Paint) — how long the largest visible element on the page takes to appear. The threshold is 2.5 seconds.

• INP (Interaction to Next Paint) — the delay before the interface responds to a user action. The threshold is 200 milliseconds.

• CLS (Cumulative Layout Shift) — how much page elements move around during loading. The threshold is 0.1.

All three are assessed at the 75th percentile of real visits — that is, on the experience of most users rather than the best possible case — and separately for mobile and desktop.

Field data and lab data are not the same thing

Field data shows how the site actually loads for real visitors on their devices and networks. Lab data is the result of a test under controlled conditions, useful for debugging a specific page. They answer different questions and may not agree — a high score in a testing tool doesn't guarantee real visitors experience the same.

What usually spoils each metric

• LCP — a slow server, the main image being discovered late, oversized files, blocking scripts, misconfigured lazy loading.

• INP — long JavaScript tasks, heavy event handlers, mass re-rendering of the interface, third-party widgets.

• CLS — images without declared dimensions, no reserved space for dynamic content, pop-up banners, fonts swapping after load.

The link between speed and sales isn't linear

Interface speed alone doesn't guarantee a rise in conversion — during optimisation several factors usually change at once: design, traffic sources, the offer. To judge the effect of speed honestly, record the specific changes, compare like-for-like audience segments, and where possible run controlled experiments rather than relying on things happening at the same time.

At Code Nova Evolution we build attention to Core Web Vitals into development rather than fixing them after the fact — if you'd like to know how these metrics look on your current site, we can run a quick audit.