The three are Largest Contentful Paint for loading, Interaction to Next Paint for responsiveness and Cumulative Layout Shift for visual stability. INP measures the full latency of an interaction, from the input event through your handler and any rendering work, until the next frame is painted. Google reports roughly the worst interaction on the page, so one slow handler ruins the score even if the average is fine.
Why interviewers ask this
INP replaced First Input Delay, so this checks that your knowledge is current rather than three years stale. The interviewer wants to hear that it includes your event handler and the resulting render, not just the delay before the handler starts, because that changes how you fix it. Knowing the thresholds and that the metric comes from real user data rather than a lab run separates people who have shipped performance work from people who have read the blog post.
How to structure your answer
- Name the three metrics and what each one covers.
- Break INP into input delay, processing time and presentation delay.
- State the good threshold and that it comes from field data.
- Say what you actually change to fix a bad score.
Example answer
The three are LCP for how fast the main content shows up, INP for responsiveness, and CLS for layout stability. INP is the interesting one because it replaced First Input Delay, and it measures the whole interaction rather than just the wait before the handler runs. So it is the input delay while the main thread is busy, plus however long my handler takes, plus the presentation delay to render the next frame. Under two hundred milliseconds is the good bucket, and it is measured from real users in the field, so it reflects mid range Android phones, not my laptop. The practical consequence is that fixing it usually means fixing the handler, not the bundle. On a search page I owned, typing was reporting around four hundred milliseconds, and it turned out every keystroke was synchronously re rendering a large result list. Debouncing the expensive part and yielding to the browser before the render dropped it under a hundred and fifty.
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See how it worksFollow-up questions to expect
- How would you find which interaction is driving a bad INP in the field?
- Why can lab tools show a good result when real users see a bad one?
- What causes a high presentation delay specifically?
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