Minecraft drops to several seconds per frame as tick uses 100% cpu

The game basically freezes up; there will be brief moments where render will happen normally for a fraction of a second, then freezes again. Interaction with the game ranges from difficult to impossible. Opening the debug screen shows the time is being taken inside root.tick.gameMode.checkedPosition < toCheckCount.unspecified.

If in a safe enough place to wait out the freeze, the game will eventually start to operate normally, then playback all the events that occurred during the freeze rapid-fire; presumably it is simply catching up on the tcp stream or something similar.