I've processed a lot of music in each of my last two roulettes, and still kept up the pace of a round per week.
I start listening immediately. I don't wait until all submissions are in. One I finish the first listen, I write one sentence off the top of my head about what I just heard.
On the second listen, I start making notes, and on my third listen, I rough out a writeup. I can generally turn most submissions around with only 4 listens, but will have some go for a 5th if they are really close.