A slow website costs you twice: visitors leave before it loads, and Google ranks it lower for the ones who stay. To measure "is this site actually fast for real people?", Google uses three metrics it calls Core Web Vitals. They sound like jargon, but each measures something you have felt yourself as a frustrated user.
This guide explains all three in plain English, tells you the scores to aim for, and — most importantly — gives you the practical fixes that move them.
The three metrics at a glance
| Metric | Measures | Good score |
|---|---|---|
| LCP — Largest Contentful Paint | Loading speed | Under 2.5s |
| INP — Interaction to Next Paint | Responsiveness | Under 200ms |
| CLS — Cumulative Layout Shift | Visual stability | Under 0.1 |
LCP: how fast does the main content appear?
What it is: Largest Contentful Paint measures how long until the biggest visible element — usually a hero image, heading, or banner — finishes loading. It is the moment a user feels "okay, the page is here."
What slows it down: huge unoptimised images, slow servers, render-blocking CSS and JavaScript, and loading fonts the wrong way.
How to fix LCP
- Optimise your images. This is the #1 fix for most sites. Compress them, serve modern formats (WebP/AVIF), and size them correctly — never ship a 4000px image into a 600px slot.
- Use a CDN so content is served from a server near each visitor.
- Preload the LCP element (e.g. the hero image) so the browser fetches it early.
- Reduce render-blocking resources — defer non-critical JavaScript and inline critical CSS.
- Improve server response time — caching, better hosting, fewer slow database queries.
INP: how quickly does the page respond to you?
What it is: Interaction to Next Paint measures the delay between a user action (a tap, a click, a keypress) and the page visibly responding. High INP is that maddening lag where you tap a button and nothing happens for half a second. (INP replaced the older FID metric in 2024.)
What slows it down: heavy JavaScript hogging the main thread, so the browser is too busy to react to the user.
How to fix INP
- Ship less JavaScript. Audit your bundle and remove what you do not need — every unused library is dead weight.
- Break up long tasks. Split heavy work so the browser can respond between chunks instead of freezing.
- Defer non-essential scripts — analytics, chat widgets, and trackers should not block interaction.
- Avoid doing expensive work on every interaction; debounce and optimise event handlers.
CLS: does the layout jump around?
You go to tap a button, an ad loads above it, everything shifts down, and you tap the wrong thing. That frustration is exactly what CLS measures.
What it is: Cumulative Layout Shift measures how much elements move around unexpectedly as the page loads. It is the most fixable of the three, and the most annoying to users.
What causes it: images and ads without reserved space, fonts that swap and reflow text, and content injected above existing content.
How to fix CLS
- Always set width and height (or aspect-ratio) on images and videos so the browser reserves their space before they load. This alone fixes most CLS problems.
- Reserve space for ads and embeds with fixed-size containers.
- Preload fonts and use
font-displaycarefully to avoid jarring text reflow. - Never insert content above existing content after the page has loaded (unless the user triggered it).
Lab data vs. field data — an important distinction
Tools like Lighthouse give you lab data: a single test in a controlled environment. Useful for debugging, but it is not what Google ranks on. Google ranks on field data — Core Web Vitals collected from real Chrome users visiting your site over the past 28 days. You can see your field data in Google Search Console under "Core Web Vitals." Optimise for the lab, but verify in the field.
Why this matters for your business
Core Web Vitals are not just an SEO box to tick. Faster sites have lower bounce rates, higher conversion rates, and happier users. A one-second improvement in load time can measurably lift sales. You are not optimising for Google — you are optimising for the human on the other end, and Google happens to reward that.
Check your scores in minutes
You cannot fix what you have not measured. siteIQ tests your Core Web Vitals along with 65+ other performance and SEO checks, then tells you exactly which images, scripts, and layout issues are dragging your scores down — in plain language, prioritised by impact. Run a free site audit and find out how fast your site really feels to your visitors.