Just ran across this very enjoyable article about how GitHub sped up cloning by several orders of magnitude – https://github.blog/2015-09-22-counting-objects/.
If you haven’t seen it, check out the git commit graph. In the Linux kernel tree, the time for me to show the commit graph of the five most recent commits went from five seconds to 20ms! https://devblogs.microsoft.com/devops/supercharging-the-git-commit-graph/. Requires Git 2.18+, which you can install on Ubuntu LTS from a PPA per this. Short version:
git config --global core.commitGraph true
and then, in each repo:
git show-ref -s | git commit-graph write --stdin-commits