I came across some situations in game that trigger heavy lag 100%:

In the Belly of the Beast: If you did the “kill Voltane” optional Rhitez will join you in the pit fight. Every time the Drow Weapon Masters start attacking Rhitez the game gets super laggy. When Rhitez is dead and the group fights the Drow Weapon Masters alone the lag disappears. The problem seems to be triggered by the Rhitez vs. hostile mobs fight, perhaps an issue with the aggro checks?

The Black Loch: Entering the cave with the pirate ships makes the game laggy, leaving the cave ends the lag.

The Shroud part 3: Even if the rest of the raid goes smoth this part is lagging.