Server Components vs Client Components: A Practical React 19 Architecture Guide
The question changed from 'is this a component' to 'where does this code run.' Here's the mental model and the mistake that quietly erases most of the performance benefit.
The Mental Model Shift: 'Where Does This Code Run' Instead of 'Is This a Component'
React Server Components changed the fundamental question developers ask when writing a component from 'what does this render' to 'where does this execute.' Server Components run once on the server and ship zero JavaScript to the client; Client Components hydrate and run interactively in the browser. Mixing them incorrectly is now the most common architecture mistake we see in React 19 codebases.
What Belongs in a Server Component
Anything that fetches data, reads from a database or filesystem, uses server-only secrets, or doesn't need interactivity belongs in a Server Component by default: page layouts, data-fetching wrappers, static content sections, and SEO-critical content that needs to be in the initial HTML response. These never ship their implementation code to the client bundle.
What Actually Requires a Client Component
Anything using React hooks (useState, useEffect), browser-only APIs, event handlers, or third-party libraries that assume a browser environment must be a Client Component, marked explicitly with the 'use client' directive. The discipline that matters: push the client boundary as far down the component tree as possible, so only the genuinely interactive leaf components ship JavaScript, not their entire parent tree.
The Performance Payoff, Measured
On the projects where we've migrated appropriately, the initial JavaScript bundle shrinks substantially because data-heavy, non-interactive sections no longer ship their rendering logic to the client: the server sends already-rendered HTML instead. This directly improves Largest Contentful Paint and Time to Interactive, the same Core Web Vitals metrics that affect both user conversion and search ranking.
The Common Mistake: Wrapping Everything in 'use client' Out of Habit
Teams migrating from older React patterns often add 'use client' to entire page trees to avoid dealing with the boundary, which silently reverts most of the performance benefit Server Components exist to provide. We audit component boundaries specifically for this pattern during code review. It's the single highest-leverage fix in a React 19 performance pass.
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