While I'm sure 100 hours a week is impossible, my dad did 6x11 work days which were dominated by coding pre-internet. You wouldn't know him, he burned out. I have personally witnessed him do more than 60 hours in a week coding. That said, his coding work isn't necessarilly creative. He can do a 12 hour stint of step through debugging stopping only to microwave frozen food. Or 12 hours of data analytic work. Since it is said this man is into optimization I'd say it is possible he's like my dad, he gets into this numb zombie state "change something, run again, look at profiler output line by line." Some people doom scroll HN or tiktok 14 hours a day, other's doom scroll a flame graph.
I also notice that the more complex a task is, the less hours of it I can do.
If I know what to write, and I just have to crunch out pretty straightforward code, I can do more hours (nowhere near 12 hours though, maybe 8 at best).
I can imagine the work your dad did, didn't include juggling a big complex system in his head, which seems to require a lot of mental energy.
That's basically also what Carmack states, that you can reach 12 hours if you plan your work to include some easier tasks for that day. But then again, I was never able to really apply that strategy.
I think it mostly takes mental energy to addapt to change. I think you can focus on a large complex mature codebase and make small improvements so long as you're not radically chaging everything. Maybe it comes down to synapses firing versus synapses rewiring but that's just a laymans guess.
