I've always wondered what exactly that "useful productivity" cap is for programmers on average. I've been gifted with the capacity for 100% productive eight hour days, but it's definitely easy to recognize that as extremely unusual talking to coworkers for more than ten minutes over the years. In discussing the idea of "maximum productive hours per day" I've found that not only is the variance on the teams I've been a part of huge, but also that it's pretty hard to correlate that number with work ethic. Great engineers often know when they're out of that maximally effective window and pivot to other things for the rest of their day.

A related aside: I've noticed anecdotally that as you become more senior you get pretty good at feeling out what that number is for each of your teammates and it becomes a pretty useful metric to leverage in your day-to-day toolkit. I'd be curious to see what a workplace would look like if folks were actually allowed to be transparent about that (without repercussions) rather than it having to be guesstimated.