Issue took place in Realms with 1.11.1 client.

Got bad omen from a captain. Walked to 2 V1 villages and no raids spawned.  Flew to a V2 village and raid started. After 3rd round, realm lagged to a frame every 30 seconds. Disconnected and logged back in, cleaned the round, and loading on the 4th round caused the same situation.  Lag to a full stop, moving a frame every 30 seconds or more.  Realms froze. Logged out and replaced with earlier backup to clean up the mess. People died as mobs kept moving and killing. Told by @Mega_Spud on tweeter to open bug report. (Thanks for your help!)