This is not really an issue but more of a suggestion. Editing very data heavy items is much easier with something like notepad, but getting the NBT is annoying (And since there is a lot of lag when opening very large items, sometimes it's impossible).

I'll add this to my list, though since multiple users have now mentioned performance issues with large items, I may also want to look into optimizing some of my code.
In the meantime, you might be able to use /nbt export file along with an external program like NBT Explorer to make editing easier than just notepad, depending on what edits you're doing.