A slow site loses its visitors before showing them anything at all. It is the most rewarding and the most neglected optimisation: we polish the design, the content, the colours, and then let a page take five seconds to appear on a phone. Speed is not an end-of-project technical detail, it is a condition of conversion and of search visibility. This guide explains what genuinely slows a site down, what Core Web Vitals are without the jargon, and the fixes that really change the outcome.
Speed is about money, not comfort
The cost of slowness is measured in lost customers. According to Google research, 53% of mobile visitors abandon a page that takes more than three seconds to load, and the probability of a bounce rises by 32% when loading goes from one to three seconds (checked on 15 June 2026). The effect carries through to the sale: a study relayed by Deloitte shows a site loading in one second converts at 3.05%, against 1.08% at five seconds (checked on 15 June 2026).
In other words, every second gained turns into visitors kept and conversions saved. Performance is not an engineering luxury, it is a direct commercial lever.
Core Web Vitals, without the jargon
Google measures the loading experience through three indicators, the Core Web Vitals. They sound technical, but each describes a concrete frustration for the visitor.
- LCP measures the time before the main element of the page appears. Plainly: how long before the visitor sees something useful. Aim under 2.5 seconds.
- INP measures responsiveness to interaction. Plainly: when the visitor clicks, does the page answer at once or lag.
- CLS measures visual stability. Plainly: does content jump around while loading, making the visitor click in the wrong place.
These three measures have become search signals. A site that looks after them pleases both its visitors and Google, which is rarely a coincidence.
What slows a site down, and how to fix it
- Images that are too heavy. Often the first cause. Compress them, choose a modern format, size them to how they are actually displayed: an immediate and enormous gain.
- Excess code and scripts. Too many extensions, fonts and third-party scripts loading before the content. Trim them, defer whatever is not essential.
- No caching. Serving a pre-built version of pages avoids rebuilding everything on each visit. One of the simplest gains there is.
- Undersized hosting. A slow server caps everything else. At a certain point, no optimisation makes up for hosting that does not fit.
- Third-party scripts left unchecked. Tracking tools, widgets, chat: each one adds weight. Keep what is useful, load the rest after the essentials.
Performance is designed in from the start, not patched afterwards. A site built light, mobile first, starts with an advantage. The subject meets our mobile-first UX good practice, where speed is treated as a feature.
The brief before the brief. Before commissioning an optimisation, measure the real state of your site on a real phone and a real connection, not on the agency fibre. A provider promising a fast site without showing you a starting measurement is selling an intention, not a measurable result.
Measure, then hold it over time
Performance is measured with free tools that score Core Web Vitals and point to concrete brakes. But an optimisation is never final: a new plugin, a heavy image added, a third-party script, and the page slows again. Speed is watched like security, continuously, not once and for all.
FAQ: website performance and speed
Why is my site slow? Most often because of images that are too heavy, an excess of scripts and extensions, no caching, or undersized hosting. A measurement identifies the dominant cause.
What are Core Web Vitals? Three Google indicators: LCP (time before the main content appears), INP (responsiveness to clicks) and CLS (visual stability). They describe the loading experience and count for search.
What load time should I aim for? Under three seconds, ideally less. Beyond that, more than half of mobile visitors give up, according to Google (checked on 15 June 2026).
Does speed affect search visibility? Yes. Core Web Vitals are ranking signals, and a slow site is also a site that loses visitors, which Google observes. Performance and search go together.
Is an optimisation permanent? No. Every addition of content, plugin or script can slow the site down. Performance is watched over time, like security.
Before you ask for a quote
Optimising the performance of a site is not a technical whim, it is a lever for conversion and for search. Start from a real measurement, deal with the dominant causes in order, and keep watching over time. Speed is designed in, it is not patched on. A deliverable is not a result.
In practice, we measure the real state of your site, identify what slows it most, and deal with the brakes in order of impact, with a measurement before and after.
