Pack Size and Clear Speeds

White mobs are the mortar that holds the more valuable mobs in place. But I've run into the same feeling. Half the time when rares are in a group of white or magic monsters, they just die to on-kill effects. This goes for bosses with minions too.
I'm currently set up to do these kinds of shenanigans with boils and herald of blood, but let me give you an example of what I mean.

I'm placing some bombs, unearthing some mobs. The screen is mostly monsters, should be threatening right? I kill something and herald of blood and the boils trigger, spreading the plague around to nearby monsters, those enemies drop blood crystals and take corrupted blood stacks, causing them to explode on death, spreading even more herald of blood effects.
So what is conceptually a difficult expedition encounter was just mostly cleared off one on kill effect.
Most rares in these kinds of situations also just die so the brick monsters if we're going back to the slightly unhinged mortar analogy from earlier just crumble over. These encounters should be the highlight of a map, but they just vanish with the filler.

But go back to that example, no aoe clear is 100% effective, there's undoubtedly some groups of monsters still alive, and each group will take roughly exactly as long to clear as the big conglomerate.

I believe there needs to be something that stops mobs from becoming so dense that clear speed does not change at all because the more mobs = more damage potential on a single target which means at least with how my build works I'm in aoe clear mode and only have to do minor cleanup. None of what these monsters do matters at this point, because what they do is less relevant than what they do when they die.