A website can have a decent PageSpeed score and still feel slow to users. This happens because performance isn’t just about one number.
One metric worth understanding is Interaction to Next Paint (INP). It measures how responsive a page is when users interact with it, such as clicking a button, opening a menu, or typing into a form.
What Usually Makes INP Worse?
One common cause is long-running JavaScript.
When JavaScript keeps the browser’s main thread busy, the browser may have to wait before responding to an interaction. This can make a page feel sluggish even when the initial page load looks fine.
Other possible causes include:
- Large amounts of JavaScript
- Expensive event handlers
- Unnecessary third-party scripts
- Heavy DOM updates
- Poorly optimised rendering
How I Would Debug It
Instead of immediately trying to improve the score, I’d first reproduce the slow interaction.
Chrome DevTools’ Performance panel can help identify what happens between the user’s interaction and the next visual update.
A simple workflow is:
Find the slow interaction → record a performance trace → identify long tasks → find the responsible code → optimise → test again.
It’s also important to compare lab testing with real-user data. A local DevTools test can help you diagnose a problem, while field data gives you a better idea of what actual visitors are experiencing.
Don’t Optimise Just for a Green Score
A green Core Web Vitals score is useful, but the real objective is a website that feels responsive to users.
Performance optimisation works better when you focus on the actual bottleneck instead of making random changes just to improve a number.
I also write about practical SEO, websites and digital strategy at hibahashim.com
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