Do AoE actually spread or is it just a visual effect?

Hello,

Do area-of-effect attacks deal all damage within a single server frame, or is there a concept of area-of-effect propagation? (In other words, do hits occur as the area expands, or is it simply a visual effect?)

It can be useful to know this in order to determine in certain cases which attack will strike first.

I'm almost certain I saw this question on the PoE 1 forum a long time ago, but I can't find it.