Web Performance: Lighthouse Metrics That Actually Matter

Google’s Core Web Vitals summarise perceived speed and stability. Lighthouse bundles lab signals — useful for debugging, but field data (CrUX) reflects real users.

Largest Contentful Paint (LCP)

Measures when the largest visible element paints. Aim for under 2.5 seconds on median hardware at mid-tier networks — optimise hero images, fonts, and server response.

Interaction to Next Paint (INP)

Replaces older “FID” style latency thinking — captures responsiveness across clicks and taps. Heavy JavaScript on the main thread hurts INP more than sheer bundle size alone suggests.

Cumulative Layout Shift (CLS)

Tracks unexpected movement — reserve space for ads, embeds, and dynamic fonts so readers do not mis-tap while scrolling.

Practical workflow

Run Lighthouse locally and in CI, but prioritise fixes that move field metrics your analytics already exposes — lab scores alone can mis-rank “feel” on cheap phones.