The server lags when a Happy Ghast is put on a lead to a fence. It only lags the server if a player is loading that specific chunk. Also, Actions & Stuff has few texture glitches with mobs. This is most likely not a Minecraft issue more so the resource pack not being updated after the latest update.
REALMS-13089
Lag on Realm
Resolved
Duplicate
2
This issue is being tracked at MCPE-222147.