> I don't think that's quite correct because there are many optimizations which are impossible (or nearly impossible) to do after a system is implemented.

Who said you have to profile after a system is implemented? Certainly I didn't: if anything, I prefer to profile during prototyping, although few companies outside the largest budget for any real prototyping these days it seems. Usually I settle for timing things and profiling as early as possible so that you can catch any performance issues before any calcifying structure is built around the non-performant code.

Yes, I did profile after the fact in the Lisp story, but my point in that story was that my reasoning led me to the wrong conclusions, not that I did everything perfectly (on the contrary, it's a story about learning from my mistakes!).

I agree that Daniel Lemire post is excellent, but nothing in that post leads me to believe he'd disagree with anything I've said.