What Are Core Web Vitals? How Many Customers Is Your Slow Site Costing You?
Google scores your site's speed and experience with three metrics: LCP, INP and CLS. What they mean, what good values look like and how to improve your site, in plain language.
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A customer clicks your ad. The page loads, and loads... By the third second their patience runs out; they hit the back button and go to a competitor. You paid for that ad, but the customer never saw your product.
Google defined three metrics to measure this experience and called them Core Web Vitals. They're part of the page experience signals considered in search ranking, and they're the most concrete indicator of how your site feels to customers.
In short:
- LCP: how fast does the page's main content appear? Good: 2.5 seconds or less.
- INP: how fast does the page respond when you click a button? Good: 200 milliseconds or less.
- CLS: do elements jump around while the page loads? Good: 0.1 or less.
- Values are assessed at the 75th percentile of real visitors' experiences, separately for mobile and desktop.
Why does speed matter so much?
Deloitte's 2020 study for Google, "Milliseconds Make Millions", found that improving mobile site speed by just 0.1 seconds raised conversion rates by about 8.4% on retail sites and about 10.1% on travel sites.
A tenth of a second. A blink takes longer.
Speed affects more than sales. A slow site sends a message about your brand too: careless, outdated, untrustworthy. Visitors feel it even if they never think it consciously.
Three metrics, three questions
Core Web Vitals map to three basic questions a user asks on arriving at a page.
LCP: "Has the page loaded?"
Largest Contentful Paint measures when the largest element in the visible area (usually the hero image, a big heading or a product photo) appears on screen. It's the moment the user thinks "OK, the page is here".
| Rating | LCP |
|---|---|
| Good | 2.5 seconds or less |
| Needs improvement | 2.5 – 4 seconds |
| Poor | More than 4 seconds |
What hurts LCP most:
- Uncompressed, oversized hero images,
- Slow server response,
- CSS and JavaScript files that block the page from showing,
- Lazy-loading the hero image.
INP: "Does it respond when I click?"
Interaction to Next Paint measures how long it takes for the page to show a visual response after interactions like clicking a button, opening a menu or typing in a form. INP replaced the earlier metric, FID (First Input Delay), in March 2024. FID looked only at the first interaction; INP accounts for interactions throughout the visit.
| Rating | INP |
|---|---|
| Good | 200 milliseconds or less |
| Needs improvement | 200 – 500 milliseconds |
| Poor | More than 500 milliseconds |
Remember pressing "Add to cart" a second time because nothing seemed to happen? That's poor INP.
What hurts INP most:
- Heavy JavaScript keeping the browser's main thread busy,
- Lots of third-party scripts (chat widgets, ad and tracking code),
- Poorly designed interfaces that redraw large parts of the page on every click.
CLS: "Does the page stay put?"
Cumulative Layout Shift measures unexpected movement of elements while the page loads. You're about to tap a news link, an ad suddenly appears above, the text shifts down and you tap the wrong link. That's CLS.
| Rating | CLS |
|---|---|
| Good | 0.1 or less |
| Needs improvement | 0.1 – 0.25 |
| Poor | More than 0.25 |
What hurts CLS most:
- Images and videos without set width and height,
- Ad slots and banners that load late and push content down,
- Text changing size when a web font loads.
What does "75th percentile" mean?
Google judges your site not by one measurement but by data collected from real users. For a page to count as "good", at least 75% of visits must fall within the good threshold. So it's not enough for your site to load fast on your fast connection and powerful computer; the experience of a customer on an old phone with a weak mobile signal counts too.
That's why mobile and desktop are assessed separately. Since much web traffic comes from mobile, your mobile scores are usually the more critical ones.
How do you measure your site?
Free, reliable tools:
- PageSpeed Insights (pagespeed.web.dev): enter your URL to see both real-user data (if there's enough traffic) and a lab test. The quickest starting point.
- Google Search Console → Core Web Vitals report: shows all pages on your site in groups, so you can see which page types have problems.
- Browser developer tools (F12) → Lighthouse and Performance tabs: for developers digging into the source of a problem.
An important distinction: lab data (a one-off test like Lighthouse) is for finding problems, and field data (real users) is for seeing the real picture. Google's assessment is based on field data. A site that scores 100 in the lab can still do poorly with real users.
Quick wins you can do today
Every site's problems differ, but these steps bring noticeable improvement on most:
- Optimize images. Convert to WebP or AVIF, and don't load images larger than their display size. On many sites this alone is the biggest win.
- Prioritize the hero image. Don't lazy-load the big image at the top of the page; tell the browser it's high priority (
fetchpriority="high"). - Give images dimensions. Set
widthandheighton every<img>or define the aspect ratio in CSS. The most common cause of CLS disappears. - Review third-party scripts. Remove tracking codes, chat widgets and plugins you no longer use. Each one weighs on INP.
- Defer JavaScript. Load scripts not needed for the first view with
defer. - Use caching and a CDN. Shorter server response time directly improves LCP.
How much do Core Web Vitals matter for SEO?
Let's be honest: Core Web Vitals are just one of Google's ranking signals. Google's own documentation stresses that relevance and quality of content matter more. A great page experience won't push weak content to the top.
But between pages of similar quality, page experience can make the difference. More importantly, the real value of Core Web Vitals is less in ranking than in conversion: getting visitors to your site is one job, keeping them there is another. For other dimensions of visibility in search and AI answers, see our SEO and GEO guide.
Frequently asked questions
My PageSpeed score isn't 100. Is that a problem?
The overall performance score (0-100) is a summary based on a lab test and isn't a direct ranking signal. What really matters is whether the three Core Web Vitals are in the "good" range in real-user data. Instead of chasing 100, focus on turning those three green.
Why don't I see real-user data for my site?
Google's field data comes from the Chrome User Experience Report (CrUX), collected from Chrome users, and isn't shown for sites without enough traffic. In that case, use lab tests as a guide.
My WordPress site is slow. Should I get a new one?
Not always. A heavy theme, lots of plugins and unoptimized images are usually the real problem, and they can be fixed. But if the foundation is fundamentally broken, or the site's business goals have changed, rebuilding may be more economical.
When will I see the effect of improvements?
Immediately in lab tests. Field data in Search Console is based on the last 28 days, so it takes about four weeks for improvements to fully show in the reports.
A fast site is the first promise you make to a customer. If you want websites and apps with green Core Web Vitals that never keep users waiting, reach us through our web application development page. Curious what happens behind the scenes while a page loads? See this post.
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