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Top 42 Web Performance Interview Questions & Answers (2026)

42 Questions 20 Beginner 12 Intermediate 10 Advanced

About Web Performance

Top 50 web performance and Core Web Vitals interview questions covering LCP, CLS, INP, rendering, caching, image optimization, and performance tooling. Companies hiring for Web Performance roles test this knowledge at every stage — from first-round screens that check the basics to final-round conversations about real-world trade-offs.

What to Expect in a Web Performance Interview

Expect a mix of conceptual and practical Web Performance questions: clear definitions and core-concept checks for junior roles, hands-on scenario questions for mid-level roles, and architecture or trade-off discussions for senior roles. Interviewers usually move from foundational topics toward the kind of problems you'd actually face on the job.

How to Use This Guide

Work through the Web Performance questions in order — Beginner, then Intermediate, then Advanced — so each concept builds on the last. Every question has its own page; bookmark the ones that trip you up and revisit them the day before your interview.

Curated by Tech Baithak Editorial Team · Last updated: May 2026

Beginner 20 questions

Core concepts every Web Performance developer must know.

01 What are Core Web Vitals? 02 What is LCP (Largest Contentful Paint)? 03 What is CLS (Cumulative Layout Shift)? 04 What is INP (Interaction to Next Paint)? 05 What is TTFB (Time to First Byte)? 06 What is the critical rendering path? 07 What are render-blocking resources? 08 What is image optimization for web performance? 09 What are JavaScript performance best practices? 10 What is lazy loading and why is it important? 11 What is browser caching and how does it improve performance? 12 What is a CDN and how does it improve web performance? 13 What is the difference between FCP and LCP? 14 What is font optimization for web performance? 15 What is preloading, prefetching, and preconnecting? 16 What are web performance metrics and tools? 17 What is code splitting and bundle optimization? 18 What is service worker caching strategy? 19 What is HTTP/2 and HTTP/3 and how do they improve performance? 20 What is GZIP vs Brotli compression?

Intermediate 12 questions

Practical knowledge for developers with hands-on experience.

01 How do you optimize React applications for performance? 02 What is the browser performance API and how do you measure performance? 03 What is layout thrashing and how do you avoid it? 04 What is resource prioritization in browsers? 05 How does SSR (Server-Side Rendering) improve performance? 06 What is a performance budget? 07 What is the impact of third-party scripts on performance? 08 What is the difference between First Input Delay (FID) and INP? 09 What is requestIdleCallback and how is it used for performance? 10 How do you optimize CSS for performance? 11 What is cumulative layout shift and how do you debug it? 12 What is server-sent events vs WebSockets for performance?

Advanced 10 questions

Deep expertise questions for senior and lead roles.

01 What is the scheduler API and how does it improve INP? 02 What is speculation rules API and prerendering? 03 How do JavaScript virtual machines optimize JavaScript performance? 04 What is the Islands Architecture and partial hydration for performance? 05 What are Container Queries and their performance implications? 06 How do you implement performance observability in production? 07 What is HTTP/3 QUIC and how does it improve performance? 08 How do you optimize the interaction responsiveness (INP) of a complex SPA? 09 What is the impact of memory leaks on web performance? 10 What are the performance implications of React Server Components?

Back to All Topics 42 questions total