A phone has a slower chip and a patchy network. Too much JavaScript, oversized images and fonts that hold back the text show up there first. Remove those three and the site gets faster.
Your developer shows you the site on a MacBook on fibre. Your customer opens it on the train, on a three-year-old phone. Those are two different websites.
A laptop on a fast connection hides almost everything. The processor is fast and the connection stable. What loads slowly, you don't see.
On a phone the opposite happens. Every kilobyte of JavaScript is downloaded, parsed and run on a slower chip. For a mobile test, Lighthouse assumes a processor four times slower. On the old A'DAM VR site the page ignored all input for 1.1 seconds in that test.
Too much JavaScript up front. Tracking scripts, chat widgets, sliders and a framework that rebuilds the whole page before anything shows. On a laptop you don't notice. On a phone you stare at an empty screen.
Images at laptop size. A hero 2400 pixels wide shown on a 390-pixel screen. The image is six times wider than needed. The phone downloads it anyway, over a mobile network.
Fonts that block everything. Four weights of two fonts, loaded from an external server, with no fallback. Until they arrive the text stays invisible.
Open your site in Chrome, press F12, pick the Performance tab and set CPU throttling to 4× and the network to 'Slow 4G'. Reload. That is roughly what your customer sees. Or simpler: run Lighthouse in mobile mode and look at First Contentful Paint and Total Blocking Time, not only the score.
We measure the way a phone loads first, not on the developer's laptop. Then we remove what doesn't need to load up front, serve images at the right size and show the text before the fonts arrive. At A'DAM VR the first text went from 7.9 to 0.9 seconds.
That a faster site automatically earns more. That depends on what happens next on the page. A visitor looking at an empty screen can leave before it loads. How many do, you measure after the change.
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