Website speed and Core Web Vitals: what you pay for twice when the page is slow

The situation

You pay €0.60 per click in Google Ads. Someone taps the ad, the page loads slowly on their phone, three seconds pass with nothing useful on screen, and they hit back. You paid for the click. You got nothing. Now multiply by a few hundred clicks a month.

This is the reality that reports hide well: speed is paid for twice. Once at the click, because Google factors landing page experience into Quality Score, so a slow page makes every click more expensive. And again at the conversion, because people who wait do not fill in forms and do not add to cart.

The three numbers that matter

Google measures the real experience of a page through three metrics, called Core Web Vitals:

• LCP (Largest Contentful Paint) — how long until the biggest visible element appears, usually the main image or headline. It answers "when do I see something useful?". Target: under 2.5 seconds.
• INP (Interaction to Next Paint) — how quickly the page responds when someone taps a button, opens a menu, types in a field. It answers "is the page listening to me?". Target: under 200 milliseconds.
• CLS (Cumulative Layout Shift) — how much things move around while loading. The button that jumps exactly when you try to tap it. Target: under 0.1.

One important detail: the thresholds are judged at the 75th percentile of real visits. Being fast on your laptop on fibre is not enough. It must be fast for three quarters of your visitors, many of them on mid-range phones, on 4G, on the tram.

How to measure properly

There are two kinds of data and they get mixed up constantly:

1. Lab data — a robot loads the page in a controlled environment and gives you a score (Lighthouse). Good as a diagnosis: it tells you what is slow. It does not tell you how your customers experience it.
2. Field data — what real Chrome users actually experienced on your site over the last 28 days. This is what counts for Google and for you.

Where to see them:

• PageSpeed Insights (pagespeed.web.dev) — enter the URL and you get both: real-user data at the top, if the site has enough traffic, and the lab diagnosis below.
• Search Console → Core Web Vitals — the report that groups your pages into "good", "needs improvement" and "poor", separately for mobile and desktop. This is where you see the whole site, not a single page.

The usual culprits

After dozens of audits, the list repeats itself:

• Images — 3 MB photos straight from a phone, not resized, no modern format (WebP/AVIF), no declared dimensions (which is also where CLS comes from).
• Third-party scripts — chat widget, pixels, maps, video embeds, "social proof" popups. Each one is small. Together they block the page for whole seconds and wreck INP.
• Fonts — four font families loaded from an external server, text invisible until they arrive.
• Heavy builders and plugins — "everything included" WordPress themes, page builders that output ten times more code than needed, 40 active plugins of which 15 no longer do anything.
• Slow hosting — the server takes 1.5 seconds to respond before anything starts. No front-end optimisation gets that time back.

What a fix session looks like

A session focused on speed runs roughly like this:

1. Baseline — field data from Search Console and PageSpeed on the 5–10 pages that receive paid or important organic traffic. Written down, so there is a "before".
2. Diagnosis — what delays LCP (usually the hero image or the server response), what blocks interaction (third-party scripts), what shifts (elements without dimensions, banners injected late).
3. Fixes in order of impact — images compressed and served at the right size, third-party scripts deferred or removed, fonts reduced and loaded properly, caching at the server or CDN level, useless plugins uninstalled.
4. Verification — the lab confirms immediately; field data confirms in 2–4 weeks.

The result is not a green score. The result is that every paid click gets a fair chance to become a customer.