Next.js 10,514 gives you a fast default, not a fast application. Server Components keep libraries off the client, prerendering removes the server from the critical path, and next/image and next/font remove two classic sources of layout shift. None of that survives a client component that imports a 700 KB syntax highlighter, a hero image with no dimensions, or a keystroke handler that filters four thousand rows synchronously. Front-End Web Development covers what the Core Web Vitals measure, Caching and Revalidation caching and Styling, Fonts, and Images images and fonts.
Every number below comes from one scratch app whose routes render the same catalog page in different ways, built with next build (Turbopack 10,514 , the default in Next.js 16.3.5) and served with next start on an Intel Core i9-7980XE at 2.60 GHz running Node.js 25.8.0 2,131 , then loaded in Chromium 153 4,389 through Playwright 26,280 at 1.6 Mbps, 150 ms of latency and a 4x CPU slowdown. They are indicative of that machine; the ratios matter more than the absolute values.