I think fixed wages have less of an effect on this because you get used to them so you don't really think about them. Especially when you aren't working hourly, the relationship between the work you do and your pay is often very tenuous. So it can feel like you are doing the work for its own sake.
For me, getting paid per hour is anathema to my enjoyment of the work. It’s a perverse incentive if I get things done faster than expected and a source of worry if things take longer. I’d much rather get paid per project, or get paid salary with TOIL for overtime worked.