hardConceptScenarioMidSeniorStaff
Field data says your page has an INP of 400 ms. How do you find the cause and fix it?
4 min readUpdated 2026-08-15
Answer
Short answer
Split the interaction into input delay, processing time and presentation delay, then attribute a real interaction from the field rather than guessing. Most of the budget is usually spent before your handler runs or after it returns, so yielding and cutting rendering cost beats optimising the handler.
Keep core web vitals explicit in the answer because that is the concept the interviewer is actually trying to test.
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Likely follow-ups
- Input delay dominates and your handler is two lines long. What is on the main thread and how do you find out?
- Why can moving work into a requestAnimationFrame callback make presentation delay worse?
- What is the difference between awaiting setTimeout zero and calling scheduler.yield, and when does it matter?
- How would you attribute a bad INP to a third-party script you do not control?
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