This is definitely one of the problems of a group like this. Lowering the number of people submitting per week and raising the word limit (which we've actually done once already--and remember, too, we're not too strict about the 5k maximum) really only gives us the same problem from a different angle. Since there usually seem to be about a dozen people active at any one time (50+ people on the actual list) that means that most people participating get to submit at least every two weeks. If we raised the word limit to 15k and lowered number of people submitting per week to, say, three people (it's five right now), then if the same number of people stay active most people will only be able to submit once a month. I would be open to trying something new if the majority of the group agrees, but it seems to me that if we went with this approach, people would ultimately only be able to get about the same amount of material through: more material at once, with a longer wait in between.
James's idea about the dropbox might work in addition to what we already do: that way, people who are hoping to push material through faster can get less detailed critiques (but that may work if you just want to catch major issues, especially if you're submitting first drafts). Other people have used beta readers as well as the RE group. You might also consider providing a partial outline or summary and only submitting the chapters you most want feedback on ... things like that. Personally, I've found this group most helpful for short fiction rather than novels for this very reason--but I recognize that only helps people who also write short fiction.
In terms of how long it takes to get longer works through the group, I'm not sure there's any real way to address it without changing the structure of weekly submissions altogether. I'd be a bit reluctant to change that (though, again, open to experimentation if a lot of people feel strongly) because it seems to work pretty well insofar as ensuring that everyone gets a decent amount of feedback each week, and I know a lot of people like having even a soft deadline. In general the weekly discussion thing seems to have worked pretty well, but it's definitely a compromise.