I didn't figure this out on my own, see: Now it's far less common for ore structures to cross chunk borders in problematic ways.

Turning down the size of our spawns (but increasing the frequency just a little to compensate) and setting coal specifically to use 'vanilla' structures instead of veins fixed this issue. With my settings, I found out that my server was sometimes stopping to generate 50 chunks because of ores when a vanilla server might have only generated a handful. But then those chunks may also have a vein that causes more chunks to be loaded. So to complete the generation of a vein of ore, the mod has to load several chunks. The veins of ore, and in my case it was almost entirely caused by massive veins of coal, were sometimes hundreds of ores long. But maybe that next chunk /also/ has an incomplete structure, so it loads a third and fourth chunk, and. This is because if a chunk is loaded, and some structure inside can't be completed without loading another chunk next to it. Vanilla Minecraft tries to avoid positioning any 'structures' such that they overlap chunk borders.

It does this chunk-by-chunk as necessary, with a chunk being a 16 x 16 x 256 section of blocks. Normally, when you explore new undeveloped terrain, the server (or internal server on singleplayer) has to generate new terrain. Turn down your coal spawn sizes, and possibly opt not to use the 'vein' ore structures. If you are getting 'cascading worldgen lag' on your server when exploring new terrain caused by this mod, consider the following:

Custom maid 3d 2 most recent version