I think biggest problem about "premature optimisation" is the problem that defining "premature" has been left to the reader and everybody has their own definiti

I think biggest problem about "premature optimisation" is the problem that defining "premature" has been left to the reader and everybody has their own definition. Then they will spar online completely needlessly because they are comparing apples to oranges. The important thing to remember is that the earlier in development something is fixed, the cheaper it gets. The longer a problem persists throughout the process, the more expensive it will be to fix. And "more" here means "exponentially more". Because at the time of design or discussing with the client I may be able to fix it in a second by just choosing a different option. Once it gets to production it might be either impossible or costs millions and months of development. The job of the person who designs it is to identify which decisions will be hard to undo later if they are found to be wrong and focus on making sure these are done correctly.