Why We Fail Our Builds on Core Web Vitals Regressions

Performance is not a design option; it is a core feature. On modern web applications, Largest Contentful Paint (LCP) and Cumulative Layout Shift (CLS) directly impact user conversion and search engine discoverability. Yet, performance regressions are notoriously easy to slip into code during day-to-day changes. A single unoptimized image, a heavy third-party library, or an unbudgeted layout shift can degrade the experience instantly.

To prevent this, we configure Lighthouse CI to assert budgets on every single commit. If a pull request causes a mobile LCP to exceed 2.5 seconds or CLS to drop past 0.1, the build fails automatically. This forces us to address performance regressions before they ever reach a staging environment or are seen by real users. It shifts quality control from reactive debugging to proactive engineering.