LCP – Largest Contentful Paint

Measures loading performance. Time for the largest visible element to render.

Good < 2.5s Needs improvement 2.5–4s Poor > 4s

Your LCP (seconds)

INP – Interaction to Next Paint

Measures interactivity. Responsiveness to user interactions (click, tap, keypress).

Good < 200ms Needs improvement 200–500ms Poor > 500ms

Your INP (ms)

CLS – Cumulative Layout Shift

Measures visual stability. Unexpected layout shifts during page load.

Good < 0.1 Needs improvement 0.1–0.25 Poor > 0.25

Your CLS score

Core Web Vitals Reference

Core Web Vitals are Google's standardized metrics for measuring real-world user experience. They directly influence Google Search rankings as part of the Page Experience signal.

The three Core Web Vitals

Metric	What it measures	Good	Needs improvement	Poor
LCP (Largest Contentful Paint)	Loading performance	< 2.5s	2.5-4.0s	> 4.0s
INP (Interaction to Next Paint)	Interactivity/responsiveness	< 200ms	200-500ms	> 500ms
CLS (Cumulative Layout Shift)	Visual stability	< 0.1	0.1-0.25	> 0.25

How to improve LCP

Optimize and compress hero images; use WebP format
Preload the LCP element with <link rel="preload">
Use a CDN; reduce server response time (TTFB)

How to improve CLS

Always set explicit width/height on images and video elements
Reserve space for ads before they load
Avoid inserting content above existing content after load

UK infrastructure context

Core Web Vitals scores are measured against real user data from the Chrome User Experience Report (CrUX), which captures actual network conditions. In the UK:

Average 4G speed: ~50 Mbps (Ofcom 2023). Fast enough that LCP is mostly decided by server response time and render-blocking resources, not bandwidth.
Slow connections: rural UK often falls to 10-20 Mbps 4G or intermittent 3G. If you target rural audiences, test on throttled 3G in DevTools. The "Good" LCP threshold of 2.5s was set with these users in mind.
UK hosting options: for UK-targeting, a data centre in London (AWS eu-west-2, GCP europe-west2, Cloudflare, Fasthosts) cuts TTFB by 50-100ms compared to US or EU servers. OVH has a UK PoP; Hetzner's nearest is Frankfurt (adding ~10ms RTT from London).
CDN impact on CLS: CDNs that rewrite HTML (some image optimisation proxies) can strip width and height attributes, worsening CLS. Test after enabling any CDN-level image optimisation.