Other claim says it has to do with adult Happy Ghasts on top of Vibrant Visuals, but the lag also happens without Vibrant Visuals.
Lag is bad for whole world when a Happy Ghast is in a loaded chunk, and especially when tethered to a post. Most tasks are slowed down (and sometimes reset to have not happened at all) including: Mining, placing blocks, eating food, and interacting with mobs.
when all Happy Ghasts in a world have been despawned, the lag is no longer noticeable.
This issue is being tracked at MCPE-222147.