Tweaking graphics settings for group play

I have most of my graphics settings turned down fairly low and the game runs well enough when I solo. But as soon as I'm in a party and a big fight goes down, the game starts really chugging and I spend most of the fight more or less frozen.

I'm guessing this is due to the extra models and their various battle effects. Are there any graphics tweaks I should be looking at to increase group play performance?

Thanks!