The issues I was having with huge TPS lag running this and netherex at the same time were resolved completely ... by me getting a better CPU (went from an Athlon II to a Ryzen 3600x) so yeah, my PC just wasn't powerful enough to run both at the same time.
Well, turns out I lied, I was testing in creative mode and everything seemed responsive. I started playing Survival and there's still some tick lag running this and netherex together. Not as much as before, it's not completely unplayable, probably because of the significant CPU upgrade, but it's definitely still there. What's weird is that a have a bunch of worldgen mods installed on the overworld and it runs just fine. Both netherex and Better Nether also run just fine when they're by themselves. I wish I knew how to pinpoint which mod is the one causing the TPS lag but neither one of them do it unless they're both installed.
Generate Overworld is easier and faster than Nether, as it has only 2 dimensions (in generator). Nether is a large cave and have height as a third dimension and, it needs to be processed from bottom to top. This needs much more time.
Generation is running smoothly, it's something else that I can't figure out that's causing the tick lag. If it was just world gen then it would stop if I'm not generating terrain, however the lag is just as prevalent in already generated areas than it is anywhere else.
Edit: I found found the problem! It is with gravestone extended, which was causing insane amounts of tick lag but only in the nether with both mods installed. For some reason.
In new version the world generator will be faster and more optimal, than now. It now bases on 3D layer system (similar to Minecraft except 3D), not on Worleys noise. It will allow defining biome size and density, and it is not repeatable. When I update it I also noticed some bugs in 1.12 version, I think I must fix them.
This biome has small lava-lakes and lava terraces, covered with some new specific plants. One of them — willow, that have blue luminescent bulbs, glowing in nether darkness. It is also a source of new sort of planks.
This biome has two subtypes — normal (with small scattered lava lakes) and terraced — with big terraces filled with lava like rice checks.