Core Web Vitals in plain English: LCP, INP and CLS

Google introduced them so that site owners had a small, consistent set of user-focused measures instead of dozens of technical ones. They are measured separately for mobile and desktop, and a page needs to pass all three to be rated “good”.

They form part of Google’s page experience signals. Google has said it rewards good page experience but still prioritises the most relevant content, so treat Core Web Vitals as a tie-breaker and a conversion issue, not a shortcut to the top.

LCP is the moment a visitor feels the page has arrived. On a product page it is often the main product photo; on a blog it might be the headline or featured image.

Common causes of a slow LCP are oversized images, slow servers, render-blocking CSS and JavaScript, and hero images that are lazy-loaded when they should load first. A hosting upgrade alone rarely fixes it if the hero image is a 3 MB photo straight from a phone.

• Compress and resize the hero image, and serve WebP or AVIF
• Don’t lazy-load the main above-the-fold image
• Use a CDN and decent hosting near your visitors
• Reduce render-blocking CSS and scripts

INP replaced First Input Delay in March 2024. FID only measured the delay before the first interaction started being handled, so most sites passed it easily. INP looks at almost every interaction during the visit and reports one of the slowest, which is much closer to what people actually feel.

The usual cause is too much JavaScript running on the browser’s main thread: chat widgets, tag managers loaded with a dozen tags, heavy page builders, sliders and analytics scripts all competing at once. On mid-range Android phones, common in India, this shows up far more than on a developer’s laptop.

Fixes are mostly about doing less work at once: remove scripts you no longer need, delay non-essential ones until after the page has loaded, and break long tasks into smaller ones so the browser can respond in between.

You have felt CLS when you go to tap a button and an ad or banner loads above it, pushing everything down. It is frustrating, and on checkout pages it causes wrong taps.

The usual culprits are images and embeds without set dimensions, cookie or offer banners injected at the top of the page, web fonts that swap in late at a different size, and ads without reserved space. Reserving the space in advance fixes most of it.

PageSpeed Insights shows both. The top section, “Discover what your real users are experiencing”, is field data from the Chrome User Experience Report. The performance score below it is a lab test and does not directly affect ranking.

Lab tools cannot measure INP properly because no real person is interacting. They show Total Blocking Time instead, which is a useful hint: high blocking time in the lab usually means poor INP in the field.

Small sites may not have enough traffic for page-level field data. In that case Search Console and PageSpeed Insights fall back to data for groups of similar pages or the whole site.

Start with mobile, because that is where most sites fail and, for many Indian businesses, where most visitors are. Open a failing group, pick an example URL and run it through PageSpeed Insights to see the likely causes.

After a fix, use “Validate fix” in Search Console. Because field data covers 28 days, it takes several weeks for the report to confirm the improvement, so don’t judge a fix after two days.