Performance3 min read
Improving Largest Contentful Paint: the four phases and where time goes
LCP is not one number, it is four phases. Optimising the wrong one is why so much performance work produces no measurable change.
Core Web Vitals are three user-centric metrics: Largest Contentful Paint (loading, good under 2.5s), Interaction to Next Paint (responsiveness, good under 200ms), and Cumulative Layout Shift (visual stability, good under 0.1). Google assesses them at the 75th percentile of real user visits, segmented by mobile and desktop.
| Metric | Measures | Good | Needs work | Poor |
|---|---|---|---|---|
| LCP | Time until the largest element renders | ≤ 2.5s | 2.5–4.0s | > 4.0s |
| INP | Latency of interactions to next paint | ≤ 200ms | 200–500ms | > 500ms |
| CLS | Unexpected layout movement | ≤ 0.1 | 0.1–0.25 | > 0.25 |
To pass, 75% of visits must be in the good range, measured separately for mobile and desktop. That percentile is why an average that looks fine can still fail — the slowest quarter of your visitors is what counts.
Lighthouse simulates one visit on a throttled connection from your machine; field data aggregates real visits across real devices and networks.
| Metric | Most common causes |
|---|---|
| LCP | Slow server response, unoptimised hero image, render-blocking CSS/JS, missing preload, client-side rendering of the main content |
| INP | Long JavaScript tasks, expensive re-renders, heavy event handlers, third-party scripts occupying the main thread |
| CLS | Images and iframes without dimensions, ads and banners injected above content, fonts causing reflow, animating layout properties |
They are a confirmed but lightweight ranking signal. Content relevance dominates. The realistic framing: passing them will not lift a poor page above a better one, but failing them costs you against comparable competitors — and it costs conversions regardless of rankings.
The commercial argument is stronger than the SEO one. Faster pages convert better, and the correlation between load time and bounce rate is consistent across every dataset we have worked with.
Interaction to Next Paint became a Core Web Vital in March 2024. FID measured only the delay before the first interaction was processed; INP measures the full latency of interactions throughout the visit, which is much closer to how responsiveness actually feels.
Field data uses a rolling 28-day window, so expect several weeks for a fix to be fully reflected. Your own RUM will show it immediately.
Per URL where there is enough traffic, otherwise grouped by similar pages. Search Console reports groups so you can fix a template rather than individual URLs.
Field data collection requires users on Chrome with reporting enabled, and covers publicly reachable URLs. Authenticated pages usually will not appear in public datasets — measure them with your own RUM.
ROVQIX Growth
SEO & growth team, ROVQIX
The ROVQIX growth team handles technical SEO, Core Web Vitals and AI-search visibility for the sites we build. Recommendations here are the ones we apply to client projects and to rovqix.in itself.
ROVQIXdesigns and builds production web platforms — Next.js front ends, Node.js APIs and the infrastructure behind them. Tell us what you're building and we'll scope it with you.
LCP is not one number, it is four phases. Optimising the wrong one is why so much performance work produces no measurable change.
INP is the metric that exposes how much JavaScript you shipped. A page can load in a second and still feel broken when tapped.
CLS is the most fixable Core Web Vital. Almost every point of it comes from an element that did not tell the browser how big it would be.
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