When editing a book, do you do it with "don't bore the reader" in mind or "a good box should (x)" in mind?

They may seem like the same thing, but the first means leaving things as they are as long as they're interesting while the second means suggesting things based on how things "should" be.

For example, a book has an opening that involves more events than it needs to. It's interesting, and something that would keep a reader reading, but the story doesn't start in earnest until the next chapter. Would you leave it as it is, or would you suggest the author cut it? Why?