Basically, from what I can tell, if you're wandering around The Other dimension, you get huge lag spikes occasionally that last for a bit. They are specific to having certain chunks loaded, and they disappear if you leave them loaded long enough, they disappear. The few times I've noticed this, there were giant pits with odd clumps of water source blocks at the top, flowing water downwards many blocks.

Edit: May also be related to sand falling.

I encountered that too and from what I've seen it's due to the huge caves under the surface which is mostly made of sand multiple layers high and with no non-falling blocks between the sand and these large open caves.

One block update to any of the sand blocks and the game almost grinds to a halt as all the sand starts falling. Making a solid layer generate between sand and caves would probably eliminate this problem mostly if not entirely.

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