High INP is not a generic “JavaScript is heavy” problem: it is one slow interaction experienced by real users. The fastest route to a useful fix is to identify which click, tap or key interaction is slow, then split its delay into input delay, processing time and presentation delay.
Start with the interaction, not the score
Use field data or reproducible traces to name the affected interaction and page state. A menu click, search suggestion and checkout button can have completely different causes.
Separate input delay from handler work
Long tasks already running before the event create input delay; expensive event handlers create processing time. Treating both as one “main-thread” problem hides the correct owner.
Inspect the long task that overlaps the event
Use a performance trace to see which script/function occupies the main thread around the interaction. Third-party tags, hydration, sliders and custom listeners are common but should be proven, not guessed.
Reduce work on the critical interaction path
Break long synchronous work, defer non-essential tasks, reduce DOM work and avoid doing unrelated analytics/rendering inside the same handler. Keep the user-visible response first.
Check presentation delay too
A fast handler can still produce slow INP if rendering/layout after the handler is expensive. Look for forced layout, large DOM updates or styles that trigger broad recalculation.
Verify the exact interaction again
Retest the same action on realistic devices and states, then watch field INP over time. A better lab trace is useful, but the real objective is faster interaction for actual visitors.
Use the real-visitors speed guide to avoid testing only an administrator session and compare with variable PageSpeed results before overreacting to one run.