I have the same issue here. When I do dungeons no problem. I have almost all the time 50-60 FPS and I am playing with all Graphic settings at max. When I do world bosses where there are more than 25-30 players, especially when bots are involving.... OMG. I have 6-8 FPS max. I do not think its my machine though because even if I lower the graphic settings its the same (10-12 FPS max). I do not know if they can fix this... but you cannot do anything in the open world when you cannot react.
For example the first world quest in the first new city of the expansion where you are waitin