Core Web Vitals explained: what LCP, INP and CLS mean for your site

01The three metrics and their thresholds

web.dev, Google's developer site, publishes the thresholds below. It also defines 'needs improvement' and 'poor' bands above each threshold; the source page has the full scale. The thresholds are applied to field data from real visitors, not to a single lab run on a fast machine.

02Why the 75th percentile matters

A page does not pass because your laptop on office Wi-Fi loads it quickly. The measurement is taken across real visits, and the value at the 75th percentile must meet the threshold, which means three quarters of visits must be at least that good. Slow phones on cellular connections count, and for a local business those are the visits that turn into calls.

That is why lab tools are useful for diagnosis but field data decides. Google Search Console's Core Web Vitals report shows field data grouped by URL; PageSpeed Insights shows field and lab data for a single page. If the two disagree, believe the field.

03What usually breaks LCP

LCP fails when the biggest thing on the screen arrives late. The usual causes are below, roughly in order of how often Redenn finds them on sites it is asked to fix.

• A large, uncompressed hero image, or a slider that loads several of them at once.
• Slow server response: an uncached PHP page, a database query on every request, or shared hosting far from the visitor.
• Render-blocking scripts and stylesheets loaded before the content.
• Web fonts that delay text until the font file arrives.
• Client-side rendering, where the page is empty until JavaScript downloads and runs.
• Autoplaying background video in the first screen.

04What usually breaks INP

INP fails when the browser is too busy to respond to a tap. The busy work is almost always JavaScript, and it is often JavaScript nobody asked for.

• Third-party scripts: chat widgets, tag managers, analytics bundles, social embeds and advertising tags competing for the main thread.
• Animation and slider libraries running on every page whether or not they are used.
• Event handlers that do heavy work, such as filtering a large list on every keystroke.
• Large component trees that must initialize before anything is interactive.
• Long tasks from tracking scripts that fire on scroll or click.

05What usually breaks CLS

CLS fails when things move after the visitor has started reading. Each cause below is a layout decision, and each has a straightforward fix.

• Images and videos without width and height, so the browser cannot reserve space.
• Banners, cookie notices or announcement bars injected above content after load.
• Web fonts swapping in and reflowing the text.
• Content loaded late (reviews, maps, embeds) that pushes existing content down.
• Animations that change layout properties instead of using transforms.

06How a static-first build helps

A static-first build pre-renders each page to HTML at build time and serves it from a CDN, so the HTML arrives without waiting for a database or a PHP process, which helps LCP. Images are sized, compressed and lazy-loaded by the framework, with dimensions fixed in the markup, which helps CLS. JavaScript is split per page and only the interactive parts ship, which helps INP.

It is not automatic. A static site with a heavy chat widget, an autoplaying video hero and three tracking scripts will still fail INP. Redenn treats every third-party script as a cost to be justified, and its pre-launch QA includes a Core Web Vitals check on real devices. Passing does not guarantee a ranking; Google's systems use many signals. What passing reliably does is stop visitors leaving before the page appears.

07How to check your own site

You do not need a developer to find out whether your site passes. Take these steps in order.

• Open the Core Web Vitals report in Google Search Console for field data across your URLs.
• Run your most-visited page through PageSpeed Insights and read the field section first.
• Test on a mid-range Android phone on a cellular connection, not on the office Wi-Fi.
• Re-check after adding any plugin, widget or tracking script; that is when scores usually drop.
• Fix the largest element first: the hero image or the slowest script is usually most of the problem.