IntermediateTECHNICAL
Describe a frontend performance problem you identified in a production app (rendering, bundle size, or runtime), the diagnostic steps you took, and the concrete optimizations you implemented and measured.
Frontend Developer
General

Sample Answer

In a consumer app with ~150k DAU I noticed slow load times: FCP around 2.8s and Lighthouse ~48. I used Chrome DevTools, field RUM, and webpack-bundle-analyzer and found a 1.2MB main bundle, heavy moment.js usage, and large synchronous chart libs blocking the main thread. I split routes, lazy-loaded charts, replaced moment with date-fns, and implemented dynamic import for heavy components. I also enabled gzip and tuned webpack tree-shaking. After deployment the main bundle dropped to ~420KB (65% reduction), FCP improved to 1.2s, and Lighthouse jumped to 85. The release reduced bounce rate on key landing pages by 18% and sped up checkout interactions for thousands of users.

Keywords

Use metrics-first diagnostics: RUM + DevTools + bundle analyzerConcrete optimizations: code-splitting, library swaps, lazy loadingMeasure impact with FCP, Lighthouse, and business metrics (bounce rate)