I play as a summon necro on single player and most of the time, I have decent framerate. Exceptions include Kurast 3000 BA and Trans Athula but that's likely because of the sheer amount of enemies and I think the Kurast 3000 Shadowgate totems also cause extra lag when my minions get close.
However, I've had two cases of serious frame drop, with probably 1 frame every 3 seconds in Hell Travincal, near Zhar the Mad and in the Judgement Day Dungeon fighting Imperius.
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I don't know how useful the picture will be but I was just setting up for the fight, with totems near Imperius and the necro curse skill then my framerate started dropping from ~30 to maybe 10 then 1 frame every few seconds.
Both instances I've had to press esc, wait a few seconds, then leave the game.
Another bug I encountered was with the same character, I was in the Duncraig or the Corrupted Abbey when I died holding a potion on my cursor. I then spam pressed esc as usual and put the potion back on my belt, I think while the respawning message was up. I then couldn't move in town, no spells worked, and drinking potions did nothing. I had to leave the game. Animations still worked, I would only be running in place or spam casting nothing.
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I suppose I might as well also ask if the potions on the belt are supposed to be so strange. I can't grab any potion off my belt unless they're in the drinking row and I can't add potions onto the belt unless they're in the drinking row or I use shift-click.