I remember that we had an similar problem when Monster used to scream their Attacks. As far as I remember that feature got removed and fixed the lag?
Maybe its something similar here?
Today it was even wors during our run. At this point, you can either ,,waste your time" and kill them one by one or in very small batches (and thus the instance will take forever, not making it worth the struggle for ,,just one coin") or die to lag and suffer an client crash 😃