React Performance: Rerender — Derived State. Keywords: derived boolean subscription. Platform: React/Next.js. Description: Subscribe to derived booleans instead of continuous values. Do: Use derived boolean state. Don't: Subscribe to continuous values. Good Example: const isMobile = useMediaQuery('(max-width: 767px)'). Bad Example: const width = useWindowWidth(); const isMobile = width < 768. Severity: Medium.