React Performance: Rerender — Transitions. Keywords: starttransition non-urgent. Platform: React/Next.js. Description: Mark frequent non-urgent state updates as transitions. Do: Use startTransition for non-urgent updates. Don't: Block UI on every state change. Good Example: startTransition(() => setScrollY(window.scrollY)). Bad Example: setScrollY(window.scrollY) // blocks on every scroll. Severity: Medium.