Core Web Vitals have been part of Google’s ranking algorithm since 2021, but they’re still misunderstood and often under-addressed across a lot of agency-built sites. If you’re handing over a new build without checking these, there’s a reasonable chance you’re delivering a site that’s already at a disadvantage in search.

This is a plain-English guide to what the metrics actually mean, how to find out where your clients stand and the most common reasons sites fail.

What Are Core Web Vitals?

Core Web Vitals are a set of user experience metrics that Google uses as ranking signals. They measure how a page actually feels to load and interact with, not just whether it loads technically, but whether it feels fast and stable from a user’s perspective.

There are currently three main metrics:

Largest Contentful Paint (LCP)

LCP measures loading performance. Specifically, it measures how long it takes for the largest visible element on the page, usually a hero image, a large heading or a video, to fully load and be visible to the user.

Target: Under 2.5 seconds.

Poor LCP is usually caused by large, unoptimised images; slow server response times; render-blocking resources (JavaScript or CSS that delay the page from loading); or slow third-party scripts.

Cumulative Layout Shift (CLS)

CLS measures visual stability. It captures how much the page layout shifts unexpectedly while loading, the experience of going to click a button and having something load above it, pushing your target down the page.

Target: Under 0.1.

Common causes include images or embeds without defined dimensions, web fonts that cause text to reflow as they load and dynamically injected content like banners or ads that push existing content around.

Interaction to Next Paint (INP)

INP measures responsiveness. It captures how quickly the page responds to user interactions, clicks, taps and keyboard inputs, throughout the entire time a user is on the page.

Target: Under 200 milliseconds.

INP replaced First Input Delay (FID) as a Core Web Vital in March 2024. Poor INP is typically caused by heavy JavaScript execution that blocks the browser’s main thread, preventing it from responding to user input promptly.

Why Do These Matter for Agency Clients?

Core Web Vitals are a confirmed Google ranking factor. Sites that perform well against these metrics have a genuine advantage in organic search over similar sites that don’t, all else being equal.

Beyond rankings, the metrics map directly to user experience. A slow, unstable or unresponsive site loses visitors. For e-commerce clients especially, even small improvements in loading time correlate with measurable improvements in conversion rate.

For agencies, there’s also a reputational angle. A site you’ve built that scores poorly in PageSpeed Insights reflects on your work. Addressing Core Web Vitals is part of delivering a site that performs, not just a site that looks good.

How to Measure Them

Google PageSpeed Insights (pagespeed.web.dev) is the quickest starting point. Paste a URL and you’ll get both lab data (simulated) and field data (real user data from the Chrome User Experience Report, if available). Field data is what Google actually uses for rankings.

Google Search Console has a Core Web Vitals report under Experience, which shows real-world performance across a site’s pages grouped into Good, Needs Improvement and Poor.

Screaming Frog (with PageSpeed API integration enabled) can pull Core Web Vitals data at scale across a whole site, which is useful for larger audits.

Note that scores can differ significantly between mobile and desktop. Google indexes mobile-first, so mobile scores are the ones to prioritise.

The Most Common Issues on Agency-Built Sites

Unoptimised images are the most frequent cause of poor LCP. Large PNGs or JPEGs, images served without compression and images not converted to next-gen formats (WebP, AVIF) all add unnecessary load time. Images should also have explicit width and height attributes set to prevent layout shift.

No lazy loading means images below the fold are loaded at the same time as images the user can actually see, adding unnecessary weight to the initial load.

Render-blocking scripts and stylesheets delay the browser from rendering the page. JavaScript that isn’t deferred or loaded asynchronously, and CSS loaded in the head without above-the-fold styles inlined, are common culprits.

Unoptimised fonts, particularly Google Fonts loaded via a standard link tag, can cause both LCP delays and layout shift as the font swaps in. Self-hosting fonts and using font-display: swap are standard fixes.

Third-party scripts (live chat widgets, analytics, ad scripts and social embeds) are often the heaviest items on a page and the hardest to control. Facade loading, where a placeholder is shown until the user interacts, can reduce the impact of things like YouTube embeds and chat widgets.

WordPress page builders, Elementor in particular, can generate significant amounts of CSS and JavaScript that is loaded globally rather than per page, adding unnecessary weight. Optimising which scripts and styles load on which pages, and using asset minification, makes a meaningful difference.

What to Do With This

If you’re an agency that builds primarily in WordPress, running a Core Web Vitals check against your recent client sites is a worthwhile exercise. You may find issues that are fixable without revisiting the design. Often the biggest gains come from image optimisation, deferred scripts and font handling rather than structural changes.

If the fixes are beyond what your team has capacity for, or sit in the grey area between developer work and SEO work, that’s where a specialist who can handle both the analysis and the implementation is useful.

Nat20 Marketing offers website performance optimisation as a standalone service alongside technical SEO audits, covering Core Web Vitals, database optimisation, plugin rationalisation and DOM size reduction. Actionable and implemented, not just reported. Get in touch to discuss.