I hope you don't mind questions from a curious visitor. I've heard so many stories about lengthy response times (overworked editors only have so many hours in the day, after all), and how a novel might see only one or two publishers in a two-year span. Your comment makes me think that the stories aren't entirely accurate. Or are agented submissions handled that much faster?
Question certainly not minded. It's a good one. With regard to the book in question -- it went to six publishers in the time period I was marketing it. Just as a point of interest. Please note that that this was one of the very earliest books I sold, so at that time I was basing it on my admittedly limited experience with other books and comparitively I was having a tough time with it. Also -- I feel the publishing landscape has changed somewhat since then. It's true that editors are now handling many more projects per capita and have less assistance to do so, thereby necessitating longer response times in many cases.
That said, your mileage will vary. One of the things I consider when submitting a project is my knowledge of the editor's reputation in terms of response time. For example, if I think there are two editors who might be interested in a book and #1 takes (on average) 3-4 months to respond, while #2 takes a year or more, I'll opt for #1 to begin. If, on the other hand, I felt that #2 was clearly the best match for the book, I'd just suck up the issue of response time and go for it. (I just had a book sell in December that had been on submission with the editor for 16 months. My goodness - I only realized that when I sat down to count it out.)
I have to admit that I don't think the times you mention are all that unusual, really. Especially if one is new to publishing or unfamiliar with editorial habits. I do think that experience has granted me something of an insight into how to get something sold faster - either by judging the tastes of the editors, the response times involved, whether I'm able to do a multiple submission, or any number of other variables. And I do think agented books get read faster. Particularly if said editor is also somewhat acquainted with the agent's taste. If the editor knows that there's a certain level of quality to be expected, it seems only practical for triage to come into play as they are far more likely to find something that works for their line from an agent who knows them and their publishing program than to take the risk of finding a diamond in the rough in the pile of unknowns.
*whew* Turned into something of an essay question. I hope that adequately answers it.