I don't think most editors WANT to sit on a manuscript. It's kind of like having the dirty dishes pile up in your sink--it's another lingering task that's left on your to-do list, reminding you of how much you have to do and making you feel guilty because it needs to get done. So you'd rather get the no out there as quickly as possible, because it actually feels good not to have things lingering (even though there's always something else waiting to linger...)
But! Sure, in the past 5 years, as publishers move to electronic submissions, and as writers move from the typewriter to the computer, it's become easier (and cheaper) than ever to write and submit a manuscript. And because it's become easier and cheaper, publishers and agents see not only an increase in submissions, but an increase in mis-directed submissions (in other words, submissions that aren't appropriate for the editor or agent at all. I get at least 2-3 YA submissions and 1 non-fiction submission a week--we publisher neither of those. And I don't get nearly the volume many publishers and agents get).
So with the increase in sheer volume, it's going to take that much longer to go through and respond. Even with a form rejection. And many editorial departments have been downsized in the past five years, so there are less editors going through more submissions.
All of those things equal a longer response time needed. I don't think most of us want to take a long time responding, but it's just a matter of not enough hours in the day--even for a profession that leaves the office and then does more work at home (which is where most editing and submission reading take place--after regular business hours, at home)
