While building a modpack for myself, I noticed a dramatic change in FPS and stability after a little while of adding mods and decided to investigate. Using VisualVM's CPU Sampler I noticed that getStackTrace() was at the top in the hotspots list and using the Find in Reverse Calls context option, I was able to see two Aether 2 events under java.lang.Thread.getStackTrace () <- com.pg85.otg.forge.dimensions.WorldProviderOTG.func_186058_p (). (I did try forward calls but they didn't seem very helpful.)

I don't know a whole lot about this sampling thing, or even what Reverse Calls are - since I can't find anything to tell me, but I found out that when I disabled The Aether 2 in the modpack, the FPS increased by at least 30 (in my 140 mod pack), there was a lot less stuttering, and the hotspot for getStackTrace() was hardly even listed.

This is where I started a controlled test with just OTG and The Aether 2.
My test consisted of standing on top of a tree in the Overworld, looking out over a roofed forest and an extreme hills with the Alt+F3 menu open. Using the same world and position for all three tests.

Here are my findings:

Mods	Avg FPS	VisualVM Snapshot
Aether2, OTG	170	-Unavailable URL-
OTG	280	-Unavailable URL-
Aether2	310	-Unavailable URL-
I just feel like that kind of difference in FPS (280 to 170) for a mod that doesn't even affect the overworld is a bit odd.

Mods Involved:

Forge	14.23.4.2747
The Aether 2	1.12.2-1.0.10
Orbis API	1.12.2-1.1.7
OTG	1.12.2 - v6

Unfortunately this is as far as my debugging skills can take me at the moment, as I'm not very familiar with Java yet and I'm not sure where else to look. But I can do my best to help as much as possible if this is, indeed, an issue.

Side note: I might be completely misinterpreting the VisualVM reports and missing the point of what is going on here, but I thought I was on to something with The Aether's onRenderGameOverlay() event since (from what I can see) it is checking for the player's dimension and OTG might deal with that kind of thing. Plus I also associate Stack Traces with errors, but that could also be wrong.
Either way, it's getting late, so here's hoping this is useful in some way :)