Core Web Vitals have been a confirmed Google ranking factor since 2021. Despite this, the majority of UK business websites still score poorly on them — and most business owners have no idea what the scores mean or why they matter — even though they directly affect how a website is designed and built. Here is a plain-English explanation of what Core Web Vitals actually measure, why they affect your rankings and your revenue, and what you can do about them.
What Core Web Vitals are
Core Web Vitals are three specific measurements of how a page feels to use from a real visitor’s perspective. They were defined by Google to create a standardised way of measuring user experience that goes beyond simple page load time.
The three metrics are Largest Contentful Paint, Interaction to Next Paint, and Cumulative Layout Shift. Each one measures something specific about how a page behaves in the browser. Each one has a threshold above which Google considers performance good, a middle range Google considers needs improvement, and a threshold below which Google considers performance poor.
Largest Contentful Paint: how fast does your page feel?
Largest Contentful Paint measures how long it takes for the main content of a page to appear in the viewport. Not the whole page — just the element that is likely to be the most visually significant thing a user is waiting for. On most websites this is a hero image, a large heading or a video thumbnail.
The threshold is 2.5 seconds for a good score. Above 4 seconds is poor. This seems generous until you understand that Google measures this on real mobile connections, not broadband. On a typical mobile connection with a typical amount of network congestion, many websites that feel fast on a desktop broadband connection score poorly on Largest Contentful Paint.
The primary causes of poor Largest Contentful Paint are server response time, render-blocking CSS and JavaScript, and unoptimised images. A slow hosting server, a page builder loading extensive JavaScript before the page can render, or a hero image that has not been compressed and formatted correctly will all push this score into poor territory.
Interaction to Next Paint: how responsive does your page feel?
Interaction to Next Paint measures how quickly a page responds when a user interacts with it — clicking a button, tapping a link, entering text in a form field. The measurement captures the time between the user doing something and the browser visually responding to it.
The threshold is 200 milliseconds for a good score. Above 500 milliseconds is poor. Delays in this range are perceptible to users even if they cannot name them. A page that feels sluggish to interact with — where buttons do not respond instantly, where there is a noticeable lag between tapping and anything happening — is scoring poorly on this metric.
The primary causes of poor Interaction to Next Paint are heavy JavaScript execution on the main thread. When the browser’s main thread is busy executing JavaScript — parsing a large bundle, running analytics, executing page builder code — it cannot immediately respond to user input. The fix is reducing the amount of JavaScript that executes on the main thread and deferring code that does not need to run immediately.
Cumulative Layout Shift: does your page jump around?
Cumulative Layout Shift measures unexpected movement of visible page elements during loading. When an image loads and pushes the text you were reading down the page, that is layout shift. When a cookie banner pops up and moves everything else, that is layout shift. When a web font loads and changes the size of headings, causing content to reflow, that is layout shift.
The threshold is 0.1 for a good score. Above 0.25 is poor. The score is dimensionless — it represents the proportion of the viewport that shifts multiplied by the distance it shifts. A large element moving a significant distance scores much worse than a small element moving slightly.
Layout shift is both a ranking factor and a direct cause of user frustration and accidental clicks. Users who are reading content that moves, or who click something they did not intend to click because the page shifted under their finger, have a poor experience that they associate with the site even if they cannot articulate why.
Why these metrics affect your business beyond rankings
The reason Google uses these metrics as ranking signals is that they correlate with actual business outcomes. Research consistently shows that faster, more stable pages have lower bounce rates, higher conversion rates and higher average order values than slower, less stable ones. The relationship is not a coincidence — visitors make rapid subconscious judgments about the quality and trustworthiness of a business based on how its website feels to use. A site that feels slow or unstable signals incompetence, whether that impression is fair or not.
A one-second improvement in page load time has been shown in multiple studies to increase conversions by measurable percentages. For an eCommerce site generating meaningful revenue, that improvement pays for itself in days. For a service business capturing leads, it means more enquiries from the same traffic.
How to check your scores
Google PageSpeed Insights at pagespeed.web.dev provides your Core Web Vitals scores using both lab data (simulated measurements) and field data (real measurements from actual visitors to your site if it has sufficient traffic). Enter your URL and look at the mobile scores specifically — these are what Google primarily uses for ranking purposes.
The results include specific recommendations for what is causing poor scores. These recommendations are technical but they identify the specific issues — render-blocking resources, image sizes, unused JavaScript, server response time — that need to be addressed.
What to do about poor scores
If your scores are poor, the fix depends on the specific causes identified in your PageSpeed report. The most common interventions are moving to faster hosting, removing or replacing a page builder with a leaner custom theme, optimising images to WebP format with correct dimensions, deferring non-critical JavaScript, and preloading fonts correctly.
For sites built on page builders, it is worth being honest about the ceiling. A well-optimised Elementor site on excellent hosting can reach the 70s and sometimes the low 80s. A well-built custom site on the same hosting can reach 95-100. If ranking performance is important to your business, the page builder is the ceiling.
We measure Core Web Vitals scores on every site we build and target 90+ on both mobile and desktop as a baseline. If you want to know where your current site sits, we will check it as part of a free website audit. If you are ready to talk about a site that performs properly from the ground up, get in touch.
Frequently asked questions
How much do Core Web Vitals actually affect Google rankings?
Google describes Core Web Vitals as a tiebreaker rather than a primary ranking signal — content relevance and authority remain more important. In practice, poor performance depresses rankings relative to competitors with equivalent content who score better. In competitive local search where content quality is similar across results, performance differences matter more.
My site looks fine to me. Does it still matter?
How a site feels on a fast broadband desktop connection is different from how it feels on a mobile device on a typical connection. Google measures the latter. Many sites that feel perfectly acceptable to owners browsing on their office WiFi score poorly on real mobile connections. Check your PageSpeed score on mobile specifically — the result may surprise you.
Will a CDN fix my Core Web Vitals?
A CDN can help with Largest Contentful Paint by serving static assets from servers geographically closer to the visitor. It does not fix render-blocking JavaScript, poor server response time or layout shift. It is one tool among several, not a complete solution.
Do Core Web Vitals affect all types of websites equally?
Google applies Core Web Vitals assessment at the page level rather than site level — pages with good scores rank better than pages with poor scores, regardless of site type. eCommerce product pages, service pages, blog posts and homepages all benefit from good scores and are affected equally by poor ones.
