Next.js 16: Turbopack, React Compiler & Cache

Next.js 15+ introduces three major shifts: Turbopack moving toward becoming the default production bundler (replacing Webpack), the React Compiler automating memoization without manual useMemo/useCallback calls, and a redesigned opt-in caching layer using explicit 'use cache' directives. Each change impacts build performance, rendering behavior, and upgrade strategy, with step-by-step migration guidance provided for all three.