After starting up a new world, I started exploring near the spawn point until a sudden crash.
This issue seems to involve several mods including: Embers, Inventory Pets, AbyssalCraft, Recurrent Complex, Astral Sorcery, denseores, and Roguelike Dungeons.

[15:10:51] [Server thread/WARN]: denseores loaded a new chunk (0, 3 Dimension: 0) during chunk population, causing cascading worldgen lag. Please report this to the mod's issue tracker. This log can be disabled in the Forge config.

I tried posting the log in zip, but it wouldn't upload.

capnkirok of Inventory-Pets seems to have the solution:

capnkirok/Inventory-Pets#170

@Boodaflow No, somebody on this thread called me out saying that capnkirok's solution is wrong... VazkiiMods/Quark#412 All of the correct methods can be found there.

I'll relay the info to capnkirok.