Treat Core Web Vitals as a design constraint, not a QA checkbox

Most rebuild briefs still treat performance as something you “optimise later.” By then the hero is a 4MB film, three webfonts are blocking paint, and the sticky booking bar jumps the layout. Google’s field Core Web Vitals — Largest Contentful Paint under 2.5s, Interaction to Next Paint under 200ms, Cumulative Layout Shift under 0.1 at the 75th percentile — are design decisions as much as engineering ones.

If you wait until Lighthouse fails before trimming motion and imagery, you are redesigning under pressure. Put a speed budget on the first viewport the same day you pick the typeface.

What that looks like in practice

Hold space for images so CLS stays calm. Prefer one strong still over autoplay video above the fold. Limit display fonts to what the brand actually needs. Keep third-party scripts off the critical path until consent — analytics that load before the paint hurt INP more than they help marketing.

Splash baselines field and lab metrics before a rebuild and publishes after numbers on case studies. If your agency cannot show LCP/INP/CLS before and after, ask why.