Top 4 Core web vitals Interview Questions
Browse 4 real-world technical and behavioral interview questions about Core web vitals. Review scenarios, edge cases, and architectural best practices.
INP went from 150ms to 400ms the week a marketing tag was added. Marketing says the script is async and cannot be the cause. How do you prove it either way and fix it?
Async controls when a script downloads, not what it does to the main thread once it executes, so an async tag can still block interaction handling. Prove it with field data attributed to the interaction: INP breaks into input delay, processing and presentation, and each points elsewhere. Then move the work off the critical path.
A page has a poor Largest Contentful Paint. How do you diagnose it and what do you actually change?
Split the LCP into TTFB, resource load delay, resource load time and render delay, use field attribution rather than a single lab run to see which part dominates, then fix that part specifically - prioritise the hero image, stop lazy-loading it, and remove render-blocking work.
Field data says your page has an INP of 400 ms. How do you find the cause and fix it?
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.
Users say the page jumps around while it loads, but your synthetic audit reports no layout shift at all. How do you find it and fix it?
Layout shift is a field problem: it depends on network speed, consent banners, ads and personalisation that a lab run never sees. Collect layout-shift entries from real sessions with their source nodes, then reserve space for whatever moved.