The bug:
The game is not multithreaded and this is very clearly a bug.
Comments 2
It's not a bug because the game IS ALREADY USING MULTITHREADING. And even if it wasn't, it would be a missing feature and not a bug. And no, Tadamichi, your comment is wrong as well.
You also can't just say "just add multithreading", that's like saying "just remove the bugs". It simply does not work like that. Please only suggest things that you actually understand. Also, this is not original at all, it's probably a duplicate of something, as well as invalid.
This is not a bug, Mojang intentionally made the game single-threaded because it only needs one thread to operate fast enough. There are mods that make the game multi-threaded though.