In my experience, it's not a lag issue. It's that the AOE indicator is a separate indicator from the actual casting bar. They are related, but not the same thing.

The AOE indicator shows you Where the attack will be.

The casting bar of the enemy shows you When the attack will hit.

The casting bar is the more important of the 2 indicators since that's what determines whether or not you get hit. As soon as you see the AOE indicator, move. Often, the casting bar finishes well before the indicator disappears.