The bug

When you're dead, entities which are removed server-side remain loaded client-side. This can cause severe lag when you're working with spawners.

Note: In recent versions this changed so entity creation is not processed either while dead. This results in massive lag when respawning.

How to reproduce

1. Set up an always active repeating command block with this command

   /summon item ~ ~1 ~ {Item:{id:stone_sword,Count:1b},Age:5999,Passengers:[{id:item,Item:{id:stone_sword,Count:1b},Age:5999,Passengers:[{id:item,Item:{id:stone_sword,Count:1b},Age:5999,Passengers:[{id:item,Item:{id:stone_sword,Count:1b},Age:5999,Passengers:[{id:item,Item:{id:stone_sword,Count:1b},Age:5999,Passengers:[{id:item,Item:{id:stone_sword,Count:1b},Age:5999,Passengers:[{id:item,Item:{id:stone_sword,Count:1b},Age:5999,Passengers:[{id:item,Item:{id:stone_sword,Count:1b},Age:5999,Passengers:[{id:item,Item:{id:stone_sword,Count:1b},Age:5999,Passengers:[{id:item,Item:{id:stone_sword,Count:1b},Age:5999}]}]}]}]}]}]}]}]}]}
   

2. Set your spawnpoint next to the command block
3. Go stand in front of the items and kill yourself
4. Wait some minutes before pressing the "Respawn" button
   After the "Respawn" button is pressed the game lags (intensity depends on how long you waited)

Another way to reproduce (LAN world)

1. Start Minecraft in 2 windows with both different names (so use one in offline mode)
2. Start a new LAN world
3. Join the LAN world with the other Minecraft window
4. Set the difficulty to peaceful
5. Press F3 and kill yourself using /kill
6. Now start spawning a zombie with a spawn egg with the player that is still alive
   → The zombies will disappear on the window with the player that is alive but they won't on the screen of the dead player. You can also see that value of "All" on the debug screen is different in the two windows.

Code analysis

Based on 1.12 decompiled using MCP 9.40 PRE 1

The code to send entity destroy packets to the client is in the method net.minecraft.entity.player.EntityPlayerMP.onUpdate() which only runs if the player is alive.

20w08a Mojang name: net.minecraft.server.level.ServerPlayer.tick()