But at the point where you're ready for careful observation and measurement, you've already designed and written your solution, and the amount of room you have

But at the point where you're ready for careful observation and measurement, you've already designed and written your solution, and the amount of room you have for optimization is constrained by the thing you just built. If you want performance, you have to design for performance, not just build the first thing that comes to mind and try to optimize it later.