[NewUser76312]

It could be something similar that we see happening in seasoned weightlifters/bodybuilders:

As your absolute strength gets stronger, the same exercises and workouts get proportionally more fatiguing.

5 sets of a bench press at an 80% of max load, taken within a rep or two of failure, done by a first-year lifter, is incredibly different from that same scheme being done by somebody who's lifted for 10 years. So more advanced lifters tend to do things like lighten the load and use variations of lifts that have more favorable stimulus-to-fatigue ratios.

Anyways, I thought maybe as an advanced programmer, something here could be analogous. You've already done all the coding and thinking to figure out easier and lower-level problems. So what you're left with are the more cognitively challenging parts of coding, which should be more mentally exhausting per unit time. Whatever is '80% difficulty' for you is probably way more advanced than what you were looking at 10 or 20 years ago.