Next.js App Router: Layouts, Pages, and Performance

Description

The provided source offers an expert-level analysis of the Next.js App Router, highlighting its re-architecture of the framework from a page-centric to a component-and-segment-centric paradigm. It explains how the App Router unifies server and client rendering, leading to significant performance enhancements and improved developer experience through file-system-based routing and nested layouts. The document details foundational concepts such as the mandatory /app directory, segment-based routing, and the roles of page.tsx for rendering UI and layout.tsx for shared UI. Furthermore, it discusses partial rendering, a key feature that optimizes user experience by preserving client-side React state during navigation, and provides a comparative analysis with the legacy Pages Router, advocating for the App Router as the default choice for new projects due to its robust and scalable architecture.