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 survival 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.