A chunk from bedrock to sky is 5KB on disk. There's no way that loading order is the problem.

There might be issues caused by the raw number of chunk loads, I'm not sure there. But I can say that the majority of my lag issues when hosting minecraft can be squarely blamed on doing world generation in the main thread.

Whick is why a lot of large servers pre-generate the world out to a few thousand blocks.

I wonder if cubic chunks is possible to add to Minecraft through modding, or to its nearest open-source 'competitor' (for lack of a better term), Minetest.

Yes, there are cubic chunks mods

https://www.minecraftforum.net/forums/mapping-and-modding-ja...

It would require a hugely significant engine rework. Networking, IO, world generation, entity AI, etc

Basically any part of the code that reads/writes chunks in the world or just references some kind of chunk-related data-type would have to be altered

Another user has posted a link to a mod for 1.12 that seems to do cubic chunks. I wouldn't have thought networking would need to be altered for servers where all users have the appropriate mods installed on the client; world generation and entity AI would follow already-established rules about how loaded chunks relate to unloaded ones.

I'd be interested in checking that out! Is it open source? Do you have a Github link?

It's very much a WIP and is not on any public git server yet. If it ever gets to that point, I'll probably post it on this site