Just a quick two cents worth.

In my experience, developers who pursue "premature optimization" usually do so to the significant detriment of maintainability.

Some developers are so focused on performance that they write code that is a nightmare to maintain for those that follow them, and ironically those that follow them often make modifications that are have significant performance costs, in large part because the "performant" code is far to clever for others to easily figure out.

So I think the phrase "premature optimization" is really more about making sure developers also focus on other really important aspects of coding instead of getting fixated on performance at all costs.

And as I said, #jmtcw.