Why run the production build before optimizing?
Because the dev bundle is large and slow, and optimizing it would mislead you. The production build is what users get, so optimize based on its output, not on the dev bundle.
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Run the production build, analyze the bundle, code split routes with React.lazy, optimize and lazy-load images, memoize only where the Profiler shows a bottleneck, remove unused dependencies, and set up caching headers.
Use a bundle analyzer. It shows every dependency and its size, so you can find heavy libraries to replace or remove, and accidentally shared chunks that could be split. Guessing wastes time.
So the initial bundle only contains what the first page needs. Users load the code for other routes on demand, which improves first load significantly, especially on slow networks.
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