I don't know if this was posted before or if it helps but I get the same lag when I solo and there are many monsters hitting me.
If I go in Reaver's Fate solo and have 30-40 elementals casting ball of lightning on me, with sent packet at 6kbps and without attacking anything, I get identical lag with the one I experience in Shroud with a full party. Same with other monsters attacking me.
I'm sure balls of lightning and monster attacks have physicial chesks too, but they are definitely not 5-6 monks and 2WF with multiple weapon effects. Also the probelm can be caused from the monsters too, not just from the player's side.
It looks like it's something else that is causing it. Or "and" something else...