INP and the New Core Web Vitals: Why a Fast Site Can Still Feel Slow
Your site loads in under a second, your Lighthouse score is green, and yet it still feels laggy when people actually use it. That gap has a name now, and Google measures it: Interaction to Next Paint, or INP.
Since INP replaced the older responsiveness metric in 2024, "fast" has needed a wider definition. Loading quickly is only half the job; responding quickly is the other half.
Load speed and responsiveness are not the same thing
Most speed advice is about how quickly a page appears. That is real and it matters — we have written about why website speed matters more than ever. But it measures the first impression, not the experience that follows.
INP measures what happens when someone interacts: taps a menu, opens a filter, submits a form. It tracks how long the page takes to visibly respond. A site can paint instantly and then stutter every time you touch it.
What INP actually measures
INP looks at the delay between a user's interaction and the next frame the browser draws in response, across the whole visit, and reports a figure that reflects the worst of it. Under 200 milliseconds is good; over 500 is poor.
The usual culprit is JavaScript. When the main thread is busy running scripts, it cannot respond to taps and clicks, so the interface freezes for a beat. Heavy third-party tags and bloated front-end frameworks are common offenders.
Why it matters beyond the score
Core Web Vitals feed into search rankings, so a poor INP can cost you visibility. But the stronger reason is trust: an interface that hesitates feels broken, even when nothing is wrong.
People rarely report it. They just sense the site is cheap or fragile, and that feeling colours how they judge the business behind it.
How to improve it
The fixes are mostly about doing less work on the main thread: trimming and deferring JavaScript, auditing third-party scripts, breaking up long tasks, and being honest about whether every interactive flourish earns its cost.
Third-party tags are often the quickest win — analytics, chat widgets and marketing scripts can quietly add hundreds of kilobytes that block interaction. Removing the ones you no longer use is free performance.
Worried your site feels slower than it scores?
We can profile your real-world responsiveness — not just load time — and pinpoint what is blocking the main thread.
If your site looks fast but does not feel it, contact us and we will take a proper look.