How to clear laggy chicken farm
Solution 1:
So far, I discovered a nice solution. I had to make a splash Potion of harming II. Then I approached the farm without looking at it (like while you play slenderman). It seems that Minecraft renderer is smart enough not to lag completelly when you do not look at the laggy area.
Solution 2:
you can go in a world editor (or NBT editor to modify the files directly) and remove all chickens from the chunk