The site was signed off in a meeting, on a laptop, on office wifi, on a large screen. It looked good.

Your customer is on a phone, on mobile data, with a data plan they are conscious of, somewhere with two bars of signal, possibly in a lift or a car park. That is the condition your site actually has to work in, and it is not the condition it was tested in.

What goes wrong first

Large images. The single biggest cause of slow pages. A photograph straight from a camera or phone can be several megabytes. Ten of those on a homepage is a page that takes an age to appear and costs the visitor real money in data.

The fix is not complicated -- resize to the dimensions actually displayed, compress, use modern formats, and load images further down the page only when the visitor scrolls toward them. This is standard practice and its absence is a sign the site was never built with the audience in mind.

Sliders and video backgrounds. A rotating banner loads several large images to show one. An autoplaying background video can be tens of megabytes. Both are expensive decorations that most visitors never watch, and on a limited plan they are actively rude.

Fonts and scripts loaded from everywhere. Each external resource is another connection, another delay, another thing that can hang. Sites accumulate these -- three analytics tools, two chat widgets, a font service, a social feed -- each added by someone who did not know about the others.

Nothing visible until everything is ready. Some sites show a blank screen or a spinner while loading. On a slow connection that blank period is where visitors leave. A page that shows its text first and its decoration second feels dramatically faster even at identical speed.

The three second reality

People abandon slow pages, and they do it faster on mobile than on desktop. The exact threshold varies, but the direction is not in doubt and the effect is largest on the first visit, from a stranger, arriving from a search -- which is precisely the visitor you paid to attract.

There is also a second cost: search engines use loading speed as a ranking input. A slow site is both harder to find and harder to keep once found.

Test it properly

Open your site on your own phone, on mobile data, with wifi turned off. Then do it somewhere with poor signal.

That is more informative than any score out of a hundred, because you experience the wait rather than reading about it. Time how long until you can read something. Time how long until you could tap the call button.

Then use one of the free page speed tools, but read the mobile results rather than the desktop ones, and pay attention to the specific recommendations rather than the headline number. The tools are quite good at telling you exactly which image is the problem.

The data question

Many customers are on plans where every megabyte is noticed. A homepage weighing several megabytes is asking a visitor to spend money to look at you.

Most of that weight is almost always images and video. A well-built page with real photographs can be under a megabyte without anyone feeling it looks cheap. The difference is entirely in preparation.

What to build instead

Text first, and text that is actual text rather than words inside an image -- readable by search engines, screen readers, and browsers with images disabled.

The important action reachable immediately. If someone arrives ready to call, they should not have to load three megabytes to find the number.

Decoration that degrades. If a font fails to load, the page should show a system font, not blank space where the words are.

And an honest audit of what is actually on the page. Most sites are carrying at least one widget nobody remembers adding and nobody looks at, costing every visitor a delay.

Why this is worth attention

Design discussions tend to focus on how the site looks when it has finished loading. For a large share of your visitors, it never entirely does. What they experience is the first two seconds.

Optimising for that is not lowering your standards. It is designing for the conditions your customers are actually in.