I had a feeling something like this was happening. When I first re-signed up for Netflix two months ago, I could send a movie out with Monday's mail and expect its replacement on Wednesday. Now, it seems, they sit on my return for a day or two before mailing out my next movie.
Case in point: I mailed back three DVDs one Monday morning. As of Tuesday afternoon, Netflix reported that it had received one of the movies and would mail the replacement that day. On Wednesday, it received the other two, one in the morning and one in the afternoon. Both of those movies' replacements were then not mailed until Friday. This happened various forms a number of times. Oh well, it's still a great value for me.
[Please note that 'heavy renters' refers to the number of movies rented, not the weight of the renter.]