4:40 i used to think POE was just kinda poorly optimized. It was laggy during combat, while stuff like D3 was flawless. That was until I started doing 4 man 150s, pulling entire maps to one spot and then having a necro area damage the pixel pull. Much worse than POE, game was barely playable.
Probably far from what you're running, but I've been having success with DD of Chain Reaction + Archmage on a mana-stacker. Instead of scaling corpse explosions, you can reliably get a ~300-500% (up to 800% if AoE overlaps) damage effectiveness per cast. I have no idea if mana-stacking is viable on a witch, however.