I think it must be at least partly personality-dependent. I know people at work who are capable of spending a week or three finding an 0.25% speed increase in our compiler, and then spending the next week or three doing it again, and so on indefinitely. Over the years those 0.25%s really add up, so it's clearly worthwhile work, but I don't think I could do a job like that: I'm just not temperamentally suited to it, because of this non-motivating feeling I get when I think about gradual quantitative improvement. I'm much happier doing a job in which I get to write clear new functionality a lot of the time. But I don't think the 0.25% people are suicidally unhappy in their work, so they must be temperamentally better suited to it than me.