Slow website: how to measure it and why it costs you clients (Core Web Vitals made simple)

A slow site goes unnoticed by whoever built it, opening it on the office computer with a good connection. It gets noticed by the client who opens it on a phone, in the car or on the train, and leaves before reading anything. Here’s how to measure speed objectively and what slows it down.

What are Core Web Vitals?

They are three indicators Google uses to measure the real experience on a page:

Google recommends treating a threshold as met when it holds for 75% of visits, with phones and computers kept separate. INP replaced the old FID metric in March 2024.

How do I measure my site’s speed?

Three free tools:

1. PageSpeed Insights (by Google): enter the address and you get both real-visit data, when available, and a lab test with suggestions.
2. Google Search Console, Core Web Vitals report: shows which groups of pages have problems, based on real data.
3. Lighthouse, built into Chrome: useful for understanding what to fix, accessibility and SEO included.

Always test on a phone: that’s where most sites struggle.

Why is a site slow? The most common causes

• Heavy images. According to Google, the largest element loaded (the one that determines LCP) is an image on the vast majority of pages. Photos weighing several megabytes, old formats, no smaller version for phones.
• Too many external scripts. Chats, maps, social widgets, tracking tools: each one adds time.
• Themes and plugins piling up. Features installed "just in case" that load code on every page.
• Badly loaded fonts, which make text appear late or jump.
• Elements without declared dimensions, which push content around while the page loads (high CLS).
• Slow hosting, or hosting far from your visitors.

How do you make a site fast?

Google’s best practices for Core Web Vitals are concrete:

• use modern formats such as WebP and AVIF, and compress without visible loss;
• serve images at the right size for each screen;
• declare the width and height of images and video;
• give priority to the main image and defer what sits further down;
• cut third-party scripts down to the ones you really need.

On an existing site, many fixes are possible. When the problem is the foundation (a heavy theme, tangled plugins), though, it often pays to start over: a site designed to be light from the start stays fast without constant maintenance. It’s one of the reasons we build custom sites in code: see custom website or template.

Does speed matter for SEO?

It’s part of page experience, which Google weighs together with many other factors: a fast site with thin content won’t overtake a useful site that’s a little slower. But at equal quality it’s an advantage, and above all it affects who stays and gets in touch. If your site doesn’t show up at all, the problem lies earlier: read why your website isn’t showing up on Google.

Want to know how your site holds up? Write to us: we’ll tell you what slows it down and whether it’s better to fix it or start over.