Found it. You're asking for too large mineshafts (MineShaftsMaxSystemSize=1280), and they are overflowing the internal cGridStructGen cache size. Currently the cache is set to 100 items, but each chunk query is asking for 196 mineshaft systems, so each chunk effectively clears the cache, and the next chunk must generate all those 196 mineshaft systems anew.
There may be additional problems with the size calculations, I'm checking those, too.
As a hotfix, try smaller system size, 128 is the default value.
There may be additional problems with the size calculations, I'm checking those, too.
As a hotfix, try smaller system size, 128 is the default value.