One more point: createrepo_c uses zstd compression level 10, but the range goes all the way up to level 22. I would oppose making the default much computationally heavier than it is currently, but if spending 20x longer to compress the repo 10% more is desirable to the fedora project, then createrepo_c could perhaps add a the ability to select a compression level.
zstd at high compression levels is very nearly as good at compressing as xz and sometimes better, while remaining much faster to decompress.