Performance work usually starts with a score, and the score is usually measured in conditions nobody experiences. A developer’s laptop is fast, its connection is fast, and by the time anyone looks at the page the browser has already stored most of it.
The visitor has none of that. First visit, mid-range phone, mobile data, somewhere with one bar.
The measurement that matters is the first one
The number worth caring about is how long it takes before the visitor can read the thing they came for, and then how long before they can act on it.
Those two are usually broken by different causes. Reading is delayed by weight — images that are far bigger than the space they occupy, fonts that block the text from appearing, scripts loaded before the content they have nothing to do with. Acting is delayed by work — a page that has arrived but is still busy assembling itself, so taps do nothing for a second or two.
The second failure is the one people describe as “it feels broken”, and it rarely shows up in a page-weight report.
The three that pay for themselves
Images, almost always. A photograph exported at full camera resolution and displayed in a card is the single most common cause of a slow page. Size them to the space, serve a modern format, and let anything below the fold load lazily.
Fonts second. A web font that blocks rendering trades a legible page for a consistent one. Showing the text immediately in a fallback and swapping when the font arrives is nearly always the better deal.
Third-party scripts third, and they deserve suspicion. Every one is a promise that someone else’s server will be fast today. A chat widget, three analytics tools and an embed can easily outweigh the entire site.
Do it on a real device
Emulated throttling in a desktop browser is a useful signal and not the truth. The truth is your own phone, on mobile data, with the site opened cold.
That test takes a minute and it changes what people prioritise, because the experience of waiting for your own page is far more persuasive than a number in a report.