Just as writers often discuss the process they have when writing, agents and editors often discuss the processes they have when editing. I think part of this discussion is the hope that we'll come up with an easier way, but in the end we're all using the process that works best for us.
The thing about the editing process that I think most authors forget is that it takes a long time. Not as long as it does to write a book certainly, but a lot longer than it takes to read one. For an editor to give a good solid edit she needs to have a little space and freedom to do so. In other words, trying to do it the week royalties are due or when there are five other things to be read won't work. Sort of like a speed edit or speedy revisions won't usually work for an author.