The attached patch removes the call to save-buffer in
org-archive-subtree. This significantly reduces the time necessary to
archive a subtree, especially when archiving many subtrees in a region.
Without this patch, Org Mode will save the archive buffer after each
subtree is archived. If one were archiving a region of 50 trees, this
would result in 50 writes to disk in quick succession. This patch makes
the behavior of org-archive-subtree match that of org-refile, which does
not save the target buffer after a refile.

Best,
Matt