What are Core Web Vitals and what thresholds define a good experience?
Assesses fundamental understanding of Web Performance conventions, runtime behavior, and memory/performance considerations.
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
Core Web Vitals are user-centric metrics for loading, interactivity and visual stability. LCP, Largest Contentful Paint, measures when the largest visible element, usually a hero image or heading, is rendered; good is 2.5 seconds or less. INP, Interaction to Next Paint, replaced FID in 2024 and measures the latency of the worst interaction at a high percentile; good is 200 milliseconds or less. CLS, Cumulative Layout Shift, quantifies unexpected movement of visible content; good is 0.1 or less.
They are evaluated at the 75th percentile of real user visits, combining mobile and desktop. They feed into Google's page experience signal and correlate with user satisfaction and conversion.
Supporting metrics include TTFB, FCP and TBT, which help diagnose root causes but are not vitals. Always optimise the tail, because a good average can hide a poor 75th percentile.
Candidate Response Strategy & Interview Tips
- Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
- Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
- Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
- Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.