Chances are good you've opened your browser's dev tools at some point, used the network panel (or perhaps even Lighthouse) to assess performance, and discovered opportunities for improvement. Then you've refactored code, improved performance, watched metrics improve, and congratulated yourself. A job well done! Only you're not done, because a problem still remains: The tests you've performed are synthetic. They don't tell you how your site is performing for real users in the field.