Can't be disclosed. People read more articles with a fast line. Having three really good feeds is optimal. Four is 2nd best. Having an extra feed smoothes expire.
Users dislike slow propagation of their own postings.
Having downstream is a bad solution because it isn't transparent to users. Iron is cheaper, NFS is more configurable. Total spool size. Annoyances to sysadmin Expire not finishing. Rare, but if it happens, it's a catastrophe.