Something must not the right...

[The Uber Cannon]

When i once got some pet Chocobos on a single-player world, i went mining. And suddenly my game lagged, i rebooted my game and it was not fixed. I checked the outside and suddenly Green chocobos were everywhere! Because though there is a creature Star Trek has which does exactly that, it clearly lagged out my game, and dropped my frames per second down to 7.
And another note, i tried ways of clearing them out.

I tried killing them normally, it was a little slow.

I tried literal cheating, it also was slow.
Any idea how this happens, and how to deal with this? Because again, i am in a singleplayer world, and i have no servers to play on. Yet this problem persists.