In one of latest libmodulemd, there is a new thing buildafter which is to some extent replacement for buildorder. Please implement it ASAP.
buildafter
Once this is done, my modules will build within 1-2 hours instead of 6-7 hours.
This issue has been migrated to Fedora Forge: https://forge.fedoraproject.org/koji/mbs/issues/1216
Please continue any further discussion there.