Fast downloads do not guarantee responsive buttons. INP investigation starts with the interactions that visitors actually perform.
Reproduce the interaction
Test navigation, accordions, search, filters and form submission on an ordinary phone. Record what was clicked and when visible feedback appeared. A spinner does not prove the underlying action succeeded, so test feedback and completion separately.
Reduce work on the main thread
Inspect long tasks and expensive event handlers. Break up suitable work, reduce unnecessary rendering and remove unused scripts. Avoid replacing one heavy library without measuring its contribution; the delay may instead be a large DOM update or third-party callback.
Protect the enquiry journey
Show immediate, accessible feedback for user actions and prevent accidental repeated submission. Test after analytics and chat widgets load, not only on a clean page. Confirm the submitted enquiry reaches its destination even when the network is slow.