It does make a good argument for just having the monsters go last every time. Or at least, having the monsters go at the same time. In other words, if there are three monsters, give them all one initiative. And then have everyone go, and then the monsters, and then everyone, repeat. It would work ok even if the monsters went in the middle of the round.
A,B, and C could act, and then monsters, and then D-F and A-C could act; repeat.
But, in general, letting people all go at the same time isn't that big of a hassle so long as people put in enough caveats (if there is room, I do X), so that tends to work as well.
DW's game runs pretty fast, since we all post pretty consistently. I've never seen it bog down.