Core Web Vitals: why a slow site costs you customers

Web performance is not an engineering topic. It is a commercial one: past three seconds of loading, a significant share of visitors leave before they have even seen your offer. Google measures that experience through three indicators, the Core Web Vitals, and factors them into ranking.

The three metrics and their thresholds

LCP: Largest Contentful Paint. Time until the largest visible element appears, usually the hero image or headline. Good: under 2.5 s.

INP: Interaction to Next Paint. The delay between a click and the page visibly reacting. It replaced FID in 2024 and it is stricter. Good: under 200 ms.

CLS: Cumulative Layout Shift. How much the layout moves unexpectedly while loading: the button that shifts just as you click it. Good: under 0.1.

These thresholds are measured on your real visitors, not in a lab. A perfect score on your office fibre says nothing about the experience on 4G in a train.

The causes we find every time

In the vast majority of audits, the same culprits come back:

• Unoptimised images. A 3 MB photo served at full resolution to be displayed in a 400 px frame. Moving to WebP or AVIF at the right dimensions often reduces the file size tenfold.
• Blocking fonts. A web font loaded without font-display: swap hides the text for the whole download.
• Third-party scripts. A live chat, two analytics tools, an ad pixel and a cookie banner: each adds tens of kilobytes of JavaScript executed before the page can respond. This is the leading cause of poor INP.
• Images without dimensions. With no width and height, the browser reserves no space: content jumps when the image arrives. That is CLS.
• Overloaded shared hosting. No front-end optimisation compensates for a server that takes 800 ms to answer.

Where to start

Order matters, because the gains are not equal:

1. Measure reality: PageSpeed Insights gives field data (CrUX) alongside the lab run. Field data is what counts.
2. Fix the images: almost always the biggest gain for the least effort.
3. Audit third-party scripts: removing one redundant analytics tool costs nothing and can deliver substantial gains.
4. Reserve space for images, iframes and banners.
5. Check server response time (TTFB) before optimising anything else.

The underlying principle

Performance is not something you catch up on at the end of a project. A fast site is one where, at every step, someone refused to add weight without a reason: one more font, a carousel library for three images, a script to animate a counter.

It is a design discipline, not an optimisation phase.