First, you are polishing writing that you might need to cut later on. This means that you are not only wasting time editorially, but making it that much harder to cut the material when so much time has been invested.
Second, you plant subconscious seeds in your early drafts. You put in things that sometimes seem to not quite belong. Yet. And if you overedit, you take them out too soon. Because later on, when you get stuck in chapter twenty-eight, if you go back and re-read what you wrote earlier,
you’ll discover you need that seed, that your subconscious put it in way back when, “knowing” you would need it.
Third, you end up spending a lot of time editing and not getting to the end of your work.
The solution: Don’t overedit your early drafts of your work, no matter how tempted you are. Move forward in the project. Don’t worry about those weird things in the early drafts that nag you. The nagging is good; you just might need those weird things later on.