This bug, along with many others I have discovered, appeared after version 1.16.100 was implemented to (from what I can tell) Xbox One users.
In a world I have, in survival mode, I attempted to summon The Wither. When I did, the boss would move very slowly in a circle and it's bar until you're able to fight it would also increase quite slowly. Once you're able to finally fight it, only The Wither will lag and lose hardly any health.