INP (Interaction to Next Paint)
How long the page takes to respond when you tap it.
What it is
Interaction to Next Paint measures the delay between a user action — a tap on a filter, a variant selector, an add-to-cart — and the screen visibly responding. It replaced First Input Delay as a Core Web Vital because it measures every interaction rather than only the first.
Why it costs you money
Poor INP is what makes a store feel broken rather than slow. The page has appeared, the shopper taps, and nothing happens for half a second because the browser is still executing JavaScript. On stores this almost always traces back to plugin and marketing scripts competing for the main thread.
How to check it yourself
Field data in Search Console reports it per page group. If it is poor while LCP is fine, the problem is script execution, not loading — start with the plugin audit rather than with caching.