How to build a high-performance SSR React app with Next.js?

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Next.js offers extensive advantages for developing performance-focused SSR (Server-Side Rendering) applications, but the differences and use cases between **SSR, SSG (Static Site Generation), and ISR (Incremental Static Regeneration)** often cause confusion. In a standard Next.js project, SSR is used by default, but you can opt for `getStaticProps` to cache pages statically or combine `getStaticPaths` with `revalidate` for dynamic regeneration. For instance, **ISR** is highly effective for product pages in e-commerce: the page is initially rendered statically and then refreshed automatically by Next.js as changes occur in the background (e.g., with `revalidate: 60`, updates happen within seconds). The impact of these approaches on **SEO** is critical—search engines index static or server-rendered content more effectively, particularly improving metrics like **First Contentful Paint (FCP)** and **Time to Interactive (TTI)**. Regarding API routes, Next.js stands out with endpoints created under the `/pages/api` directory, offering end-to-end type safety and minimal structures. For example, when writing a RESTful API route, you can directly add TypeScript type definitions to the `req` and `res` objects, significantly reducing error rates. Additionally, leveraging the Edge Runtime (e.g., running on Cloudflare Workers) allows you to globally distribute your APIs, ensuring low-latency responses regardless of geographic location. **Another marginal benefit** comes from middleware: by defining operations like token authentication or URL redirects in a `middleware.ts` file before page loads, you can prevent unnecessary server requests. These optimizations, especially for protected routes, directly correlate with performance gains.