Unless you're a senior editor with intern-like beings below you on the food chain who open and process the slush for you to look at-a splendid luxury!-a substantial fraction of your time is going to go into opening the packages, logging in the name, title, agent/no agent, genre, and date rejected, and then repackaging the rejected manuscript with a form rejection letter and a copy of the Submission Guidelines.
Manuscripts are unwieldy, but the real reason for that time ratio is that most of them are a fast reject. Herewith, the rough breakdown of manuscript characteristics, from most to least obvious rejections:
1. Author is functionally illiterate.
2. Author has submitted some variety of literature we don't publish: poetry, religious revelation, political rant, illustrated fanfic, etc.
3. Author has a serious neurochemical disorder, puts all important words into capital letters, and would type out to the margins if MSWord would let him.
4. Author is on bad terms with the Muse of Language. Parts of speech are not what they should be. Confusion-of-motion problems inadvertently generate hideous images. Words are supplanted by their similar-sounding cousins: towed the line, deep-seeded, dire straights, nearly penultimate, incentiary, reeking havoc, plaintiff melody, viscous/vicious, causal/casual, clamoured to her feet, a shutter went through her body, his body went ridged, empirical storm troopers, et cetera.
5. Author can write basic sentences, but not string them together in any way that adds up to paragraphs.
6. Author has a moderate neurochemical disorder and can't tell when he or she has changed the subject. This greatly facilitates composition, but is hard on comprehension.
7. Author can write passable paragraphs, and has a sufficiently functional plot that readers would notice if you shuffled the chapters into a different order. However, the story and the manner of its telling are alike hackneyed, dull, and pointless.
(At this point, you have eliminated 60-75% of your submissions. Almost all the reading-and-thinking time will be spent on the remaining fraction. )
8. It's nice that the author is working on his/her problems, but the process would be better served by seeing a shrink than by writing novels.
9. Nobody but the author is ever going to care about this dull, flaccid, underperforming book.
10. The book has an engaging plot. Trouble is, it's not the author's, and everybody's already seen that movie/read that book/collected that comic.
(You have now eliminated 95-99% of the submissions.)
11. Someone could publish this book, but we don't see why it should be us.
12. Author is talented, but has written the wrong book.
13. It's a good book, but the house isn't going to get behind it, so if you buy it, it'll just get lost in the shuffle.
14. Buy this book.
Aspiring writers are forever asking what the odds are that they'll wind up in category #14. That's the wrong question. If you've written a book that surprises, amuses, and delights the readers, and gives them a strong incentive to read all the pages in order, your chances are very good indeed. If not, your chances are poor.
I frequently see denunciations from writers who say an editor can't possibly judge their novel from three chapters and an outline. Sure we can, even if the chapters are short and the first one's atypical. In many cases, three pages are enough. You don't have to drink the entire carton of milk in order to tell that it's gone bad. And in any event, three chapters are certainly long enough to tell you whether you want to look at the rest of the book.
But let's assume the author's right, and the reader didn't get all the way through the submitted material. Is that a fair evaluation? When we're publishing books that readers are going to glance at, briefly browse, then either buy or put back on the shelf, you bet it's a fair evaluation. Again, when you think about this with your reader-mind instead of your writer-mind, it all comes much clearer.