Playing in vanilla version 1.16.1, player's behavior creates lag when they tried to open chests in ocean ruin, whatever it's single player or on server.

1. The chests that caused lag we have found all contains a burried treasure map. (Since they were from the same ruin, the map points towards the same treasure.)

2. For paper servers, such lag will cause the server to terminate itself. For official server it would not, but player will be kicked out.

3. When more chests were opened, it is less likely to cause lag.

4. Example: Map seed: 522582913814293307, chest position: 693,63,4537, 716,48,4530