Slideshow FPS while mining 2 km tunnel at bedrock (Navezgane)

IMPORTANT: I know there isn't a pastebin link, the log file is just over the limit. I have included it along with the world itself in a Google Drive link below in place of the pastebin link.

Summary:

A friend and I were mining a 2 km long tunnel at bedrock from our base at Trader Joel toward the Pass N' Gas Station to the East. At some point, he begins experiencing less than 5 - 10 FPS, thinking it was his PC having been on for 12 hours. Meanwhile, my computer on 12 year old hardware that's been on for 3+ days is doing just fine. That was until shortly after he hopped off and I started heading back to our base (at around 5:23 AM in the logs). 0.8 FPS. I have absolutely no idea what could be causing this, my friend reducing his LOD Distance to 0% also did nothing.

I have a feeling it either has to do with culling, or a dozen or more screamers that spawned after we went to town with our augers. The strange thing to me is the fact it was so delayed and didn't start happening until I made my way back. Come to think of it, I don't think it started happening to my buddy until he did the same (returning from the tunnel to base).

Game Version: A21.1 b16 (Stable)

OS/Version: Windows 10

CPU Model: Intel i7 965 @ 3.2 GHz

System Memory: 24 GB DDR3

GPU Model and VRAM: Nvidia GeForce GTX 680, 2 GB VRAM

Screen Resolution: 1920x1080, Fullscreen

Video Settings: Lowest settings, LOD Distance at 100% (Framerate is almost always 60+ when not near large cities)

Gamemode: MP Host (Steam) on Navezgane

Did you wipe old saves? No

Did you start a new game? No

Did you validate your files? No

Are you using any mods? No, but if modlets (XML-based mods) count, then yes. I am using the following:
- The Freehold's Heatmap reduction for workstations and tool / weapon noise, roughly 10% and 50% of originals respectively
- Donovan's vehicle tweaks
- Donovan's more books in loot
- Gazz' A21 loot bag timer extension

- MeanCloud's enemy health bars

- Red Eagle LXIX's "Add Land Claim Count Option" for the world menu
(All of these are QoL features I would love to see added to the vanilla game, but modlets are a perfect compromise since my friends don't even have to download them)

EAC on or off? On.

Status: NEW

Bug Description: Major FPS drop at bottom of world in Navezgane

Detailed steps to reproduce the bug:

1) (I've included my world file below for anyone looking to test for themselves.)

1.1) Download the world and load it in-game

2) Go to Trader Joel's, at his front gate is my base. On the North-facing wall (away from Trader Joel's) inside the walled lot with farmland, there is a steel hatch. Down there is our "mine" which is being turned into the aforementioned 2 km tunnel. Go down there.

3) At around 400 - 500 meters away from the bottom of the ladder are two minibikes we use for transit. A couple dozen or so meters away from these (moving toward the ladder / exit) is where my framerate finally got back to normal. Going through either of the two smaller tunnels though, you might end up in a 1920's silent film. My buddy and I made multiple trips to and from, so you might have to do the same.

Actual result: I'm experiencing framerate akin to Let's Game It Out's infamous Satisfactory "Beltnado" while trying to make my way back.

Expected result: My framerate to... Not do that, I guess?

~~Link to Pastebin Output Log:~~ (Log file too big, uploaded to Google Drive instead)

Link to Output Log and World File: https://drive.google.com/drive/folders/1uEeHt0dynD2WKaNvBxA2DbJPd3pIWyWa