Website speed: how to measure it and improve it

Website speed is one of the few things whose impact is measurable in two places at once: a visitor leaves a slow site before seeing the content, and Google factors speed into its rankings. In both cases the business loses customers it never even saw.

The good news: speed is a technical problem with technical causes. Unlike many marketing questions, here you can measure, fix, and measure again.

What Google actually measures

Google uses three metrics known as Core Web Vitals. Behind them is not an abstract notion of "speed" but what the visitor actually experiences.

LCP (Largest Contentful Paint) — how quickly the largest content element appears, usually the heading or the hero image. A good result is under 2.5 seconds. This is the most important metric, because it measures the moment the visitor understands that the page has arrived.

INP (Interaction to Next Paint) — how quickly the page responds to a click or a tap. A good result is under 200 milliseconds. If a menu opens half a second later, the site feels broken even if it loaded quickly.

CLS (Cumulative Layout Shift) — how much the content jumps around while loading. A good result is under 0.1. The classic case: you tap a button, but a banner loaded in a moment earlier and the tap went somewhere else.

Alongside these, TTFB (the server's first response) matters too — if the server is slow to answer, every other metric is already lost.

How to test your own site

PageSpeed Insights (pagespeed.web.dev) — paste in a URL and you get both a lab measurement and real-user data where enough of it exists. Always look at the mobile view, because most visitors arrive on a phone and the mobile result is almost always worse.

The Core Web Vitals report in Search Console shows real-user data and which groups of pages are problematic. This matters more than the score from a single test.

Browser developer tools (the Network panel) show which file exactly is slowing things down. Sort by size — the answer is usually in the first three rows.

One warning: do not chase 100 points. The PageSpeed score is an estimate, not a goal. What matters is that LCP, INP and CLS stay in the green — 90 points with a green LCP is better than 98 points without one.

The five most common causes of slowness

1. Unoptimised images

By far the most frequent cause. A photo from a phone camera is 4–8 MB and 4,000 pixels wide; on screen it is displayed at 800 pixels. The visitor downloads ten times more data than they see.

What to do: resize images to the size they are displayed at, use modern formats (WebP, AVIF), set width and height on images (this immediately improves CLS), and defer images below the fold with loading="lazy".

2. Too many plugins

A typical WordPress site runs 25–40 plugins, and each one adds its own CSS and JS files to every page, including pages where it is never used. The contact form plugin loads its script on the front page, the slider plugin on the contact page.

What to do: remove unused plugins entirely (not just deactivate them), replace several plugins' functions with one, and check whether any plugin loads external fonts or scripts. When the plugin count becomes a three-digit problem, it is a question of architecture rather than configuration — see WordPress vs a code-based website.

3. Slow or overloaded hosting

Cheap shared hosting means hundreds of sites on the same server. If a neighbour gets a traffic spike, your site slows down. A TTFB above 600 ms almost always points to hosting or the database.

What to do: measure TTFB, check the PHP version (an old PHP is several times slower), enable server-side caching and gzip/brotli compression. If the site matters commercially, better hosting is the cheapest speed gain available.

4. Too much JavaScript

Every script has to be downloaded, parsed and executed — in the same thread that draws the page. Analytics, chat widgets, cookie banners, social buttons and A/B testing tools all stack on top of each other.

What to do: load third-party scripts with defer/async, drop scripts nobody looks at (if nobody ever writes in the chat widget, its 200 kB is not justified), and check whether analytics is loaded more than once.

5. Web fonts

A font loaded from an external source means an extra connection and often a moment where the text is invisible. That damages LCP and CLS at the same time.

What to do: host fonts on your own server, use font-display: swap, load only the weights you actually use (not nine when you use three), and subset the character set.

What to do if the site is already slow

A practical order that gives the most result for the least effort:

1. Measure first. Write down LCP, INP and CLS on mobile. Without a baseline you will not know later whether anything improved.
2. Fix the images. This alone usually delivers half the win.
3. Check TTFB and hosting. If the server answers slowly, nothing further helps.
4. Prune scripts and plugins. Remove everything that has not been needed once in a week.
5. Measure again and only then move on to fine-tuning.

If a site is built so that every view is assembled from a database and ten plugins, the ceiling arrives quickly. We build sites where every page is pre-generated — the server calculates nothing at the moment of the visit, there is no database, and the attack surface is many times smaller. The same principle is spelled out in migrating from WordPress to a modern site.

Does speed actually bring customers?

Honestly: speed alone does not. If the site cannot be found or its message is unclear, a 1.2-second LCP saves nothing — we have written about that in why does nobody visit my website.

But speed is an amplifier. Once a site brings visitors, every second saved means a larger share of them reach the content. And unlike content and links, it is a one-off job whose result persists.

Summary

Speed does not have to be guessed — it can be measured. Three metrics (LCP, INP, CLS), one free tool and five typical causes cover the overwhelming majority of cases, and images alone often solve half of it.

If you want to know where your site stands and what specifically slows it down, order a free website audit — we go through speed, technical SEO and user experience and tell you what to tackle in what order. For a new site, see the website development page.