Being a founder isn’t about writing code all day - you don’t have to be operating at full mental capacity to be getting stuff done. You spend a ton of time talking to people, recruiting, writing emails, etc. IMO the notion that you can be more effective as a founder in < 8 hrs/day as if you work 8 hrs/day is a pleasant sounding falsehood.

I’m sure there are diminishing returns at some point, but I don’t think it’s before 8 hrs/day.