Everyone agrees a website should be fast, but “fast” is vague. Google measures it with three numbers called Core Web Vitals. Here is what they mean, what to aim for and why most slow websites are slow, without the jargon.

Why speed matters

People are impatient on their phones. A widely quoted 2016 Google study found that 53% of mobile visits are abandoned if a page takes longer than three seconds to load. Speed also plays a part in search: Google says its ranking systems use Core Web Vitals as one of many page experience signals. Good content still matters most, but when two pages are similar, the faster one has an edge, and it usually converts better too.

The three numbers Google uses

  • Largest Contentful Paint (LCP) measures how long the main content takes to appear. Good is 2.5 seconds or less.
  • Interaction to Next Paint (INP) measures how quickly the page responds when someone taps or clicks. Good is 200 milliseconds or less. INP replaced an older measure called First Input Delay in March 2024.
  • Cumulative Layout Shift (CLS) measures how much things jump around while the page loads, like a button moving just as you tap it. Good is 0.1 or less.

Google judges each number on how your real visitors experience it, and looks at the 75th percentile: three out of four visits need to meet the target for a page to count as good.

How to check your own website

Paste your address into Google’s free PageSpeed Insights tool. The top section shows real-visitor data when Google has enough of it; the section below is a lab test you can rerun as often as you like. If you use Google Search Console, its Core Web Vitals report shows which groups of pages need work.

Why most slow websites are slow

  • Huge images. A 4 MB photo straight from a phone camera, shown at the size of a postage stamp.
  • Too many add-ons. Chat widgets, sliders, trackers and plugins, each adding its own scripts.
  • Distant, overloaded hosting. A cheap shared server on another continent, shared with thousands of other sites.
  • Heavy fonts. Several font families in many weights, loaded before any text appears.
  • Things that jump. Images and banners without fixed sizes that push content down as they load.

Quick wins

  • Resize and compress images, and use modern formats like WebP or AVIF
  • Remove plugins and scripts you are not sure you need
  • Serve the site from a content delivery network close to your visitors
  • Use one or two font families and load only the weights you use
  • Give every image and embed a fixed width and height

What we aim for

Google’s “good” threshold is a minimum, not a goal. On the websites we build, we aim for the main content to appear in about a second on a mid-range phone, by pre-building pages, sending very little JavaScript and serving everything from a global network. This website is built the same way, so feel free to test it.

If your website is slow and you are not sure why, our website redesign service starts with a speed audit that shows exactly where the time goes.