# Slow site, lost customers: Core Web Vitals explained

> Google measures how fast your website loads, how quickly it responds and how stable it stays. What Core Web Vitals mean and how to check your own site.

Simplix · performance, seo, websites

Have you ever opened a website, gone to tap a button and, at that exact moment, the page shifted and you tapped something else? Or waited a few seconds in front of a blank screen and closed the page? Your customers do the same.

Google measures these things on the real visitors of every website, with three indicators called **Core Web Vitals**. You don’t need to be a programmer to understand them, or to check your website.

## The three measurements

| Indicator | What it measures, in short | Good | Poor |
| --- | --- | --- | --- |
| **LCP** (Largest Contentful Paint) | how long it takes for the page’s main content to appear | under 2.5 s | over 4 s |
| **INP** (Interaction to Next Paint) | how quickly the page reacts when you tap, type or open a menu | under 200 ms | over 500 ms |
| **CLS** (Cumulative Layout Shift) | how much the content “jumps” while loading | under 0.1 | over 0.25 |

A website passes if at least **75% of visits** are rated “good” on all three indicators[^1]. The measurements come from real visitors using Chrome, not from a lab test, so they reflect your customers’ actual phones and connections.

> **FID has been replaced**
>
> If you read about Core Web Vitals a few years ago, you may remember FID. Since March 2024 it has been replaced by INP, which measures the page’s responsiveness throughout the whole visit, not just on the first tap.

## Why it matters

**For Google.** Google says explicitly that its ranking systems use Core Web Vitals. It says just as clearly that a good score doesn’t guarantee the top spot: relevant content comes first[^2]. In other words, between two equally good pages, the fast one has an edge.

**For customers.** Here the effect is more direct. Vodafone tested two versions of the same page, identical in appearance: the one with a 31% better LCP brought **8% more sales**[^3]. A Deloitte study for Google found that, for online stores, a page just a tenth of a second faster increased conversions by 8.4%[^4].

**Against the competition.** According to the 2025 Web Almanac, only **48% of websites** pass on mobile[^5]. They fail most often on LCP: the main content shows up too late. A fast website already puts you ahead of half your competitors.

## How to check your own website

1. **Open [PageSpeed Insights](https://pagespeed.web.dev/)** and enter your website’s address (in full, exactly as it appears in the browser’s address bar).
2. **Look at the top part first**, with the data from real users. That’s where you see whether the website passes the Core Web Vitals assessment, on mobile and desktop.
3. **Only then look at the score below** (0–100). It’s a lab test, run on the spot, useful for finding problems, but it isn’t what Google uses for ranking.
4. **If you have Google Search Console**, the “Core Web Vitals” report shows which pages have problems and of what kind.

> **No data from real users?**
>
> Websites with few visits don’t always have enough real-world data. In that case, the mobile lab score is the best indication. A score above 90 is good; below 50 means your visitors really are waiting.

## What usually hurts the score

**When LCP is poor** (content appears late):

- large, unoptimized images, especially the main image at the top of the page;
- sliders that load five images to show one;
- slow hosting or a server too far from visitors;
- fonts and scripts that block the page from displaying.

**When INP is poor** (the page reacts slowly):

- too much JavaScript: heavy themes, dozens of plugins, visual page builders (for example Elementor, Divi, WPBakery, or platforms like Wix and Squarespace);
- third-party scripts: chat, ad pixels, maps, social media widgets;
- menus and filters that recalculate the whole page on every click.

**When CLS is poor** (content jumps):

- images and videos without declared dimensions;
- banners, ads or pop-ups appearing above content that is already displayed;
- fonts that swap after loading and move the text.

Many of these are relatively easy to fix. Others, such as a theme that loads a megabyte of JavaScript on every page, belong to the website’s foundation and can’t be fixed without rebuilding part of it.

## How we build fast websites

Speed isn’t added at the end, like a coat of paint. It is decided from the start, by the way the website is built:

- pages are generated on the server, as clean HTML, without waiting for JavaScript (we avoid frameworks like React, Angular or Vue used to build the page in the browser, which load a lot of JavaScript up front);
- images are automatically converted to modern formats (AVIF and WebP), in several sizes, so a phone downloads only what it needs;
- fonts are hosted on the same server and reduced to the characters needed;
- there is little JavaScript, loaded only where it’s needed;
- every image has declared dimensions, so the page doesn’t jump.

![Lighthouse report for the Simplix home page, on mobile: 100 for performance, accessibility and best practices](https://www.simplix.ro/assets/images/blog/core-web-vitals/lighthouse-simplix/lighthouse-simplix-1600.webp "Lighthouse report for our home page, on mobile: the main content appears in 1.4 seconds and the page barely moves while loading")

The result shows in the report above: 100 for performance on mobile, the main content in 1.4 seconds and a page that stays put. We test every new page the same way before publishing it.

Want to find out why your website is slow and what can be fixed?

[Request a performance audit](https://www.simplix.ro/en/services/it-consulting-and-infrastructure/)

[^1]: web.dev, [How the Core Web Vitals metrics thresholds were defined](https://web.dev/articles/defining-core-web-vitals-thresholds).
[^2]: Google Search Central, [Understanding page experience in Google Search results](https://developers.google.com/search/docs/appearance/page-experience).
[^3]: web.dev, [Vodafone: A 31% improvement in LCP increased sales by 8%](https://web.dev/case-studies/vodafone).
[^4]: Deloitte and Google, [Milliseconds make millions](https://web.dev/case-studies/milliseconds-make-millions), 2020.
[^5]: HTTP Archive, [Web Almanac 2025: Performance](https://almanac.httparchive.org/en/2025/performance), data from July 2025.