Replacing existing mobs (without crashing the game!)

So, I was trying to replace some of MC's existing mobs with unique new variants. I tried to use the "On entity joins world" procedure, so that - for instance - every zombie that joined the world would be replaced with [insert mob here]. However... every time I started up the mod after adding such a procedure, all the mobs were stationary and I couldn't break blocks or use items, etc., most likely on account of the fact that the game was checking every entity in existence over and over to determine if they needed to be replaced. Either way, the game was unplayable as a result :(

So, is there a better way (using the existing procedures system, preferably) to implement such a thing?