Aggro type is a very intresting topic. Depending on the method you use it can take at minimum 2 seconds to even start to move or 1 second. The other gold farm that I linked to and said not to build uses a very odd aggression system. It first uses turtle eggs(the slowest aggro method) to lure the pigmen down a fall. Then they have to walk another long distance to get to the player before they die. This makes the time to kill very slow which increases the mob cap which I talked about in the all mob farm section. It would be much more ideal to have them fall a little further onto a block that breaks their fall like a honey block. Then just put a kill chamber there and it is already much more efficient. The best aggro method is using a lama. You build a structure around it and a dog's structure(i will not be going into detail about what that is) and the lama shoots the pigmen. They then spring after 1 second of spawning towards the lama and fall down.