Dieu recently added an improvement that made mods insta kill after 5 seconds. However, this already shows flaws: limited time to kill (since there are grinders that have mobs pushed somewhere), inability to afk and wait for the mobs to stack, and less enjoyable grinding. I recommend implementing the old feature of mobs where they can stack on a multiplier. Not only would this solve the issue of there being several mobs (and lag) but it also still adds that feeling of grinding souls/farming. For instance:
https://imgur.com/a/ql3KukA . Please consider this as this is effecting the gameplay of survival.