Aggy, that may be true of some markets. (I believe that Stupefying Stories, specifically, has said that they either do that or have done it in the past.) It may even be true of Apex.
However, at least as far as Apex goes, I'm virtually certain it's not the whole story.
I sent a story to Apex on June 12th with my usual minimal cover letter. I received a reply in six days. I submitted another story on June 21st and have not heard back yet. Admittedly, it's *possible* that my most recent story has been held for further review, but when I look back at my other responses, the patterns (or lack thereof) seem to fit more closely with inconsistent performance across slush readers for that market. (For example, my two quickest responses -- neither of which would've been for a romantic YA western
-- both came from the same slush reader.)
I don't necessarily think of this as a conspiracy theory, but as the explanation that makes the most sense to me for the data I have at hand. (Which, I suppose, is probably what most people who believe conspiracy theories thing. Stop looking at me like that!)