Steps (no single "code" solution — this is a measurement exercise):

1. In any project from this course, run:
     npm run dev
   Open the app, open React DevTools' Profiler tab, record a short
   session while performing a typical interaction (e.g. clicking a
   button that updates a list), then stop recording and note the
   reported render durations for the components involved.

2. Build and preview the production version instead:
     npm run build
     npm run preview
   Open the URL printed by `preview` (a real production build, not
   the dev server), repeat the exact same interaction, and record
   another Profiler session.

3. Compare the two recordings' render durations for the same
   components doing the same work.

Expected observation:
  - The dev-server recording will almost always show noticeably
    longer render durations for the same components and the same
    interaction.
  - This is because the dev server includes React's extra development-
    only checks (prop-type validation, double-invoking certain
    functions to catch side effects, more verbose internal bookkeeping)
    that exist purely to give better warnings/errors while coding —
    none of which run in the production build.
  - Conclusion: any performance number gathered from `npm run dev`
    should never be treated as representative of what a real user
    actually experiences. Always profile `npm run preview` (or an
    actual deployed build) before deciding something is "too slow."