All this zone lag going on because a world boss just happens to be in that zone, and this isn’t the first time as this has happened in other expansions, and nothing ever gets done about it.
If you can’t stabilize servers for big world bosses, then stop adding them.
Also the heroic world tier really doesn’t help this lag problem.
Because now the world boss has largely increased HP and therefore the fight goes on a lot lot longer, prolonging the lag.
So don’t even think of adding this mode to retail, if you can’t make it lag free then just don’t.