Github usage predates the Gitlab instance and some projects haven't been migrated. We also keep some repositories on Github as a secondary mirror.
We've also had some performance issues on Gitlab for `svntogit` repository because of weird branch structures. However this repo is not deprecated so it doesn't matter anymore.
Seems like you're using btrfs for repos.archlinux.org. Any problems with reliability and/or performance? Very rarely does someone share their experience if everything is working fine, regardless of a technology in question.