One of the best tips I can give new copy editors is the following.
Instead of going through a file and dealing with each issue as you find it, do this: batch every task.
But what do I mean by “batch every task”?
For example, deal with all the chapter titles in one go. And don’t get sidetracked if you see something else that needs doing as you work your way through the chapter titles.
Then go over just the main chapter subheads.
Then tackle the chapter sub-subheads. See what I mean?
Multitasking is a bad way to work.
If you try to focus on, say, three different tasks at the same time, you end up doing none of them well. And it takes you longer to get the job done.
Instead focus 100% of your attention on just one task and you’ll (1) work faster, (2) get less tired mentally, and (3) make fewer mistakes.