For both trash clearing (e.g. MH) and Felmyst, I suggest you set the following to minimum:

1) Spell effects
2) Shadow
3) Terrain distance
4) Terrain detail distance (forgot what it's called)

For Felmyst specifically, try to look downwards as much as possible during dusting phase.


Edit:
Also, if you haven't, please delete your WTF folder and all your non-Blizzard mods. Reinstall the post-3.02 versions of the mods.

This post has been edited by Aggroboy: Oct 23 2008, 09:07 AM

A lot of the spikes are actually our PCs choking at the graphics, because the effects are pretty heavy during massive AOE or Felmyst.

Old outdated mods or WTF configs cause a lot of lag also. It took a while for me to convince stubborn people to refresh their WTF and interface folders, but the effects are noticeable.