In my very personal experience, many times startups are not full of the brightest engineers, especially if they are based in a major tech hub such as the Bay Area: most talented engineers these days, if location is not an issue, first try a shot at FAANG or similarly big companies where they can experience problems at massive scale and very high compensation.

Hence, many folks who end up accepting to work at a startup are less talented engineers (you're left with the "scraps" of the market), so if you're a high performing individual you might find that you really don't need to put that many hours in to be effective, as compared to your peers (and that's deeply depressing as well, and one of the reasons why I left startups for FAANG, other than compensation; talent is SO SO SO much better).

I speak for direct experience, I rode a startup train for a long time at a company that grew a lot, and rarely put in more than 40 hours a week into the job, while others were routinely putting 60+. And I kept being praised by the management and technical leadership.

The problem (lack of local talent due to big company competition) became severe enough that we had to bootstrap multiple remote teams in easily overlooked areas because the new local hires (San Francisco) were literally trashing the product due to poor development/testing.