Web Performance Interview Questions
Performance questions separate senior candidates. Here are the ones that come up and how to answer them.
What are Core Web Vitals?
Google's user-centric metrics: LCP (load, target < 2.5s), INP (responsiveness, replaces FID), and CLS (visual stability, target < 0.1). Know what improves each.
How do you improve load performance?
Code splitting and lazy loading, smaller dependencies, critical CSS inlined, image optimization (AVIF/WebP, responsive srcset, lazy), and preloading critical resources and fonts.
What are reflow and repaint?
Reflow recalculates layout (expensive); repaint redraws pixels (cheaper). Batch DOM reads then writes to avoid layout thrashing, and animate transform/opacity, not layout properties.
How do you optimise a long list?
Virtualization (windowing) — render only the rows in the viewport and recycle them on scroll, so performance is independent of total data size.
How do caching layers help?
Browser HTTP cache (immutable hashed assets), CDN at the edge, an app cache (React Query stale-while-revalidate), and Service Worker/IndexedDB for offline — each with a clear invalidation plan.
Ace the performance round
Core Web Vitals, loading and runtime performance, caching and virtualization — in the Complete Frontend Kit.
⚡ Get the Complete Frontend Kit → ₹499Frequently asked questions
- How do you measure performance?
- Lighthouse and the Performance panel for lab data, and RUM (field Core Web Vitals) for real users. Optimise the proven bottleneck.
- What's the most common performance mistake?
- Premature optimisation — memoizing everything without measuring. Profile first, then optimise the real hotspot.