Next.js 15 introduces significant architectural changes with the App Router and Server Components as defaults. Server Components render on the server first, improving initial load times and SEO, while Client Components handle user interactions using the 'use client' directive. The new version offers better performance through reduced JavaScript bundle sizes, enhanced image optimization, and improved data fetching capabilities. The App Router provides cleaner file organization and routing management, making it easier for developers to build scalable web applications with optimal performance and user experience.