Ok so basically using a hacked client my friend, surrounded himself in charged respawn anchors and then repeatedly used as many of them as he could, constantly. at first, there wasn't much lag and it was just really loud. but after a while, all of the constant modification to the player's NBT to be able to set his spawn point caused the server to hang. this can be done in a singleplayer or on any server on 1.16 that hasn't fixed the issue through plugins