I started my world in 1.13.2.
I converted it and did a backup before using the latest 18w43b snapshot.
I was flying around making everything was working and noticed there was slight lag / entity lag (stuttering)
I flew over to the dark oak forest surrounding my base (Loaded in 1.13) to find it overflowing with mobs.
It only appeared to be happening in the dark oak forests, not sure if it is affecting every biome.
I am not 100% sure if the lag is because of their spawning, but it appears to be.