I posted this patch to rpm-metadata-list, but I figure some folks here
might be interested as well.
http://people.redhat.com/mikem/software/createrepo-update2.patch
This patch allows createrepo to recycle existing metadata so that it
only has to scan the rpms that have been changed or added. For
situations where you might want to make small incremental changes to a
yum repo (e.g. mock), this can provide significant time savings.