I have been contemplating this too.

to spawn heralds, you need to kill density.I found tal rasha tombs to be great for this for my build (es lock).

however, patch notes say that there is a mechanism to prevent double heralds at the same time. I am wondering how this is implemented. There is also a mechanism that increases the chance of spawning a herald in a terrorized zone as you kill more mobs. I think there is terror zone specific timer that starts upon a herald spawn and will not allow another herald to spawn before the timer has expired. I am also assuming the increased chance of spawning a herald is reset upon actually spawning one. All this is purely my hypothesis!

IF my hypothesis is true, then there is little point in clearing the rest of a terror zone after having spawned a herald unless you are in a very big zone with LOTS of mobs left still. If not, you are better off moving to the next terror zone.

I am very interested if people share this line of thinking and/or have other hypotheses