I'm going to remove the crowbar since we already have a few, it's no different. I'm keeping the propane tank since it's unique.
I never said the L4B/ZAPT AI was advanced :/ Or are you just mentioning this?
I still strongly think the way they are coded to spawn and move in L4B and ZAPT is the reason behind why so many lag.
I literally tested this in Single Player with 20 Zombies - the default max - killing bricks in their path, coming towards me, no problems.
That also reminds me, most of the time when the zombies collide with you, they do not damage you. I need to fix that.