xJon/Hexxit-II#177 I posted here first before I determined it might be a Dimensional Doors mod problem.

I used timings to show that there are 16287 dropped items in a chunk when having the bad performance.
I did a command to kill dropped items due to the server going to high CPU usage and 10+GB of RAM. Most of the result showed: item.item.world_thread which seems to be related to the Dimensional Doors mod https://ftb.fandom.com/wiki/World_Thread .

I am unsure if it happened in previous versions but I did not notice this occurring before when I was on DimensionalDoors-3.0.9-287 so I believe this is a new problem now that I upgraded to DimensionalDoors-1.12.2-3.0.12 along with the Hexxit II modpack update 1.4.1. I do not see any errors in the logs dealing with Dimensional Doors.

It's probably due to the fact that wild rifts eat blocks and drop world thread now. You can actually specifically disable dropping world thread from the config until I figure out how to fix that

@TheComputerizer who generously worked on the last few patches of the mod for 1.12.2 might have a better idea what's happening, although could also be caused by #297 or a SpongeForge incompatibility?