Description of the issue:

mineralVeins are saved in a single file, not in chunk data, as the world gets larger and larger this becomes a performance issue and leads to server freezes when the world is being saved

To reproduce, simply add this file to a worlds data folder, and watch how long world saves take.

Vanilla had a similar issue with structure data in older versions: https://bugs.mojang.com/browse/MC-33134

Versions & Modlist

Forge 35.1.13
IE 1.16.4-4.1.2-129