this can cut down bandwith usage and storage space by around 40%:
170744 -rw-rw-r-- linux-firmware-20181218.tar.gz
115896 -rw-r--r-- linux-firmware-20181218.tar.xz
We are pulling from upstream's cgit instance which doesn't support .tar.xz.
Does upstream have a bugtracker or mailing list, where this can be discussed?
When I made .xz snapshots, I used git archive on a local checkout of the linux-firmware tree. It is also possible to download the snapshot, decompress and recompress using xz if the committing developer deems desirable.
Upstream could enable xz compressed snapshots by adding "snapshot tar.xz" to cgitrc. However generating xz rather than gzip increases system load considerably, so I doubt they are inclined to.
I got a response from one of the admins of kernel.org, they are going to produce precreated tarballs with cryptographically verified signatures using the tar.xz format in early February. They will be made available via www.kernel.org/pub/
Ok, so since half a year has passend and the tar.xz archives which were promised by upstream are still nowhere to be found, I'm going to close this as cantfix.