I used to play on a 1.7.10 modpack, and while it was a little unstable, the only thing that really affected me was blaze fireballs. I'd have a consistent 60 fps until blazes would shoot at me, and then my fps would instantly drop to single digits or less. I chalked it up to a fluke of the modpack.
Now, I've moved on to 1.12.2, and I've noticed that the Crimson Cultists in Thaumcraft also use the same fireballs, and they lag just as much. It becomes literally unplayable. My FPS becomes less than 1. I get a frame update every other second. And then it goes away as if it never happened, until they shoot fireballs again. Interestingly, the Basalz from Thermal Foundation shoots rocks in the exact same way, but these don't lag. It's just fireballs. I've tested in vanilla 1.12.2 also, and the blaze fireball lag is still there.
I wouldn't think it's an issue with my hardware being inadequate because it can handle everything like a champ. But it's this one issue that persists across game versions, modded or not, that completely cripples the game.