Frontend: Diagnosing Slow Core Web Vitals

The product team reports poor Core Web Vitals — especially LCP and CLS — after a release. Walk through your diagnosis.

Technical Reference & Key Concepts

The Challenge: Core Web Vitals Regression

A recent release made the app noticeably slower to paint and the layout visibly shifts while loading. The team asks you to own the fix.

Core questions to address:

  1. How would you confirm and measure the regression (which metrics, which tools)?
  2. What are the most likely culprits for a bad LCP and for CLS, in order of likelihood?
  3. What would you ship as the fix, and how would you prove it worked?