Open DevTools right now. Go to the Network tab, find the throttling dropdown, and set it to Slow 3G. Now reload something you built.
Watch the blank white screen sit there for a few seconds. Watch the layout jump around as fonts and images land out of order. Somewhere in the world, someone is looking at that exact screen on that exact connection, right now, and they don't have a throttling dropdown to turn back off.
"Optimize later" is one of the most common plans in web development, and it's also one of the most misleading. It suggests performance is a separate phase, something you schedule after the feature is done, the way you'd schedule a coat of paint after the walls go up. In practice it rarely gets scheduled at all. The feature ships, the next feature starts, and the page just keeps getting a little heavier, a little slower, until a Lighthouse score forces the conversation nobody planned for.
The developers who never have that conversation aren't smarter or more disciplined in some abstract way. They just built a handful of habits into how they write code every day, so performance was never a separate phase to schedule in the first place. None of what follows costs extra time once it's automatic. Most of it saves time, because catching a heavy dependency while you're still deciding whether to add it is faster than ripping it out after six other things depend on it.
