How does next/link make navigation fast? Explain client-side navigation and prefetching.

next/link swaps pages in the browser without a full reload, and it fetches pages before the user clicks.

Client-side navigation. <Link> renders an a tag. But a click does a client-side navigation instead of a full page load:

• Next.js fetches only the RSC payload for the new route (the compact description of its rendered Server Component tree).
• Shared layouts stay mounted, with their state.
• The URL and browser history update.

Prefetching. In production, links are prefetched automatically when they scroll into view.

• For static routes, the whole route is prefetched.
• For dynamic routes, Next.js prefetches the shared layout and the loading.tsx fallback. So the loading state shows at once.

Prefetched data is kept in the browser Router Cache. You can turn prefetching off with prefetch={false}. Prefetching does not run in development.

In code. For programmatic navigation in Client Components, use useRouter from next/navigation. Its router also has router.prefetch().