It's really not slow right now. Publishers have slightly more slots to fill this year than they had last year, or the year before that. Slots have to be filled, regardless.
When a manuscript sits too long, it's because no one with any clout is nudging the editor. It's a mistake to think all agents, even selling agents, have any more clout than the average unpublished writer.
Even fifteen years ago, I knew of cases where manuscripts sat for up to two years because the editors had no reason not to let them sit. If it's a book an editor thinks he might want at some point, assuming the right slot opens, or when the particular market for that book gets hot, but it isn't a book the editor thinks readers will jump all over, he has every reason to sit on it, and no reason to accept or reject it it, until and unless he gets the right pressure.
Pressure can come from an agent with clout, one who has sent the editor a number of books that sold very, very well, or it can come form a request to withdraw the book. But if it doesn't come, why should the editor be in a hurry? He's still getting paid every week, and he won't make a dime more if he does or doesn't sit on a book he might want somewhere down the line, but that he doesn't think is going to set any sales records right now.
On the other side of the hand, no writer should be waiting for anything. He should be writing a second, or even a third or fourth novel, and add these to the books he has in submission. It's tough to control anything finished and submitted, but it's easy to control how much more you write, and how many more projects you get into submission while the fate of a first novel is being decided.
