INP

What it is

A visitor taps something. The browser has to finish whatever it was doing, run your event handler, recalculate the page and paint the result. INP is the time from the tap to that paint, and the score a page gets is close to its worst interaction over the whole visit.

That last part is what makes it hard. FID, the metric it replaced, timed only the delay before the first interaction was handled — a page could pass FID and still freeze for half a second every time someone opened a menu. INP watches all of it.

Why it matters if you are paying for a website

Because it measures the feeling people describe as “the site is laggy”, which is the one performance complaint visitors actually voice. A page can paint in a second and still feel broken if the first tap on the menu takes 400 ms to do anything.

What actually moves it

Main-thread work, and almost nothing else. Three sources, in the order I usually find them:

Third-party tags. A tag manager container with six things inside it, a chat widget, a heatmap recorder. Each one runs JavaScript on the same thread that has to answer the tap. This is the single most common cause on business sites, and it is a decision the marketing department made, not the developer.

Hydration. A page that ships a whole framework so it can make a component interactive has to boot that framework before anything responds. The fix is architectural: send HTML, and hydrate only the parts that genuinely need it. That is why this site’s interactive pieces are islands rather than a single application — the menu, the lightbox and the theme toggle carry their own small scripts, and the article they sit in carries none.

Work done in the handler. A click listener that recalculates a filter over 500 items before it paints anything. Paint first, compute after.

What INP is not

It is not measurable in a lab in any way you should trust. Lighthouse reports a proxy, because nobody clicked anything during the run. Real INP comes from real sessions — the Search Console report, or a field-data tool — and until a page has traffic there is no honest number to quote.