Three seconds, and the customer is gone
A visitor landing on a page does not wonder whether it is well coded. They wait for it to appear, and if the wait drags on, they leave, often before they have even seen what you had to sell them. This behaviour is widespread and well documented: a large share of users abandon a page that takes more than a few seconds to become usable, and that share rises quickly on mobile, where most traffic now comes from.
Slowness is therefore not a minor inconvenience to be fixed one day when there is time. It is a silent leak: potential customers who will never see the offer, baskets that will never fill, an advertising budget spent bringing visitors to a page they leave straight away. The worst part is that it does not show up in your analytics as a problem, only as a high bounce rate whose cause you look for elsewhere.
What Google actually measures
Google has ended up putting figures on this experience through what it calls the Core Web Vitals. These metrics do not judge how attractive a site is or how rich its code is, but what a user actually feels: how long before the main content appears, how quickly the page responds when you interact with it, and whether elements jump around unexpectedly while it loads.
Since March 2024, one of these metrics has changed. Responsiveness is no longer measured on the first click alone but by INP, which assesses how lively the page is throughout the visit. In other words, a site that seems to display quickly but struggles as soon as you use it no longer gets away with it. These metrics count twice over: they weigh on rankings in search results, and they describe exactly what makes a visitor stay or leave.
The three indicators and the thresholds Google considers good:
LCP (Largest Contentful Paint), the time taken to display the main content: under 2.5 seconds.
INP (Interaction to Next Paint), how responsive the page is to interactions: under 200 milliseconds.
CLS (Cumulative Layout Shift), visual stability during loading: under 0.1.
Mobile sets the rules
These requirements make full sense on mobile, now the main way into most websites. A smartphone has less processing power than a desktop computer, a connection that can become patchy as soon as you move away from good coverage, and a screen where the slightest layout shift is obvious. A page judged fast on the designer's workstation can become painful on a train or in a poorly served area, precisely where the customer was trying to place an order.
Image weight is often the first culprit. Visuals uploaded at full resolution, without compression or adaptation to the screen size, are enough to slow an entire page on a mobile network. Next come the scripts accumulated over the years, each added for a good reason, but which together end up overwhelming the visitor's browser. Optimisation almost always starts by removing what is superfluous before adding any fix at all.
Performance is built in at the design stage, not bolted on
Faced with a slow site, the temptation is to pile on fixes: a caching plugin, image compression, a tool that promises to speed everything up in one click. These measures sometimes help, but they treat the symptoms. A site built on a heavy foundation, overloaded with scripts and features nobody uses, will remain fundamentally slow whatever varnish is applied to it. This is particularly true of some installations where an accumulation of extensions ends up smothering the page.
Real performance is decided upstream, in the initial technical choices. A site that sends the browser pages that are already built, stripped of unnecessary code and served from a network close to the user, starts with a lead that nothing can make up later. That is the logic of a modern approach such as Astro combined with edge hosting: generating fast static pages rather than recalculating every page view. Speed is not a setting added at the end; it is a property of the foundation.
Third-party scripts deserve particular attention here, because they often escape the site's control while weighing heavily on its loading. An analytics tool, a chat widget, an advertising banner or a video player each bring their own code, loaded from an external server you have no control over. Added together, they can on their own double the display time of an otherwise well-built page. Every service added should therefore justify itself by what it actually brings in, because performance is a budget spent with every extra building block, whether you see it or not.
Measure before you decide
Before investing in optimisation, you need to know where you stand, and on this point intuition is often misleading. A site can seem fast from the managing director's computer, on a fibre connection with a browser that has cached everything, and crawl on a customer's smartphone over 4G. Google's measurement tools give an objective reading of that gap, distinguishing lab data, useful for diagnosis, from field data collected from real visitors, which reflects the actual experience.
This diagnosis is also the best basis for making decisions. It turns a vague complaint, "the site is slow", into concrete priorities: this image is too heavy and delays the main content, that third-party script blocks responsiveness, this block loads late and makes the layout jump. You then know what to fix, and above all what it will bring in.
Speed is a commercial investment
Improving a site's performance is not about satisfying some abstract technical requirement. It is about winning back visitors that slowness was driving away and giving Google one more reason to rank your pages well. On an e-commerce site, a few tenths of a second saved show up directly in the conversion rate.
The effect can be measured on both sides of the equation. On the acquisition side, a faster, better-ranked site attracts more visitors for the same advertising budget, since every euro spent brings people to a page they no longer leave immediately. On the conversion side, studies of online retail show that cutting load time leads to a measurable rise in sales, precisely because you stop losing buyers while they wait. Performance is one of the few projects that acts at the same time on the number of visitors and on the share who go on to buy, which makes it a profitable investment well beyond browsing comfort alone.
That is why we treat speed as a prerequisite from the start of any website creation project, not as an option to add later. When an existing site has plateaued, it is also one of the levers we examine during a redesign that keeps your search rankings.
Do you feel your site is slow, or would you like to know what it really costs you in lost visitors? Our website audit measures technical, mobile, SEO and conversion issues before prioritising the fixes.
Sources: Google web.dev, Core Web Vitals · Google, INP replaces FID