I've noticied that the world generates new terrain very slowly when I have Realistic Ore Veins, and have it configured with this datapack

Do you have any ideas as to how I can profile this issue, to find the cause? :)

Datapack in zip file for convinience:
oreveins.zip

Edit: I take it back I'm an idiot. Threw on Visual VM + your data pack to see what's going wrong and it's all in vanilla biome generation calls. So there's something I can try and improve and see how much of a difference that makes.

I've explained the gist of this before here, but the main idea is that this mod is designed for cascading lag free ore generation. That is a problem which is only ever going to be present if ore generation is already causing cascading lag, i.e. your veins are larger than a chunk (size of > 16 blocks). In order to avoid this, the ore generation is going to be, by necessity, slower than comparable vanilla generation for smaller ore veins. Looking through your configurations, I saw a lot of high count, and low rarity, resulting in trying to generate many veins per chunk, and a lot of small veins as opposed to a few large ones.

So, if your primary purpose is to generate ores that are similar to vanilla in size or rarity, this is not a mod that is designed for that, and is always going to be worse than vanilla or similar style generation for that purpose.

That said, I'm sure there are avenues for performance improvements within realistic ore veins, however I'm unaware of anything I can actually do without doing some deep digging into performance and actually start properly profiling ore generation with a test set such as the one you have.

TL;DR, in meme format,

Excellent meme answer :P

Okay, I've fixed the glaring issue I noticed, let me know if it's still awful or if that helped at all.

It helped a ton, I can now fly in creative without ever reaching "blank" terrain - Thanks a lot!