Missed opportunity to switch to zstd IMO. Much faster than gzip, especially during decompression, and essentially the same compression ratio. Them apt upgrades take too long.
But then with deltas, mirror sizes grows even more, which is a hard sell.
Arch Linux gets away with using zstd -21 but that's not practical for general purpose Linux distributions, as it has significant memory requirements compared to xz.
Also the speedup in practice is negligent - while zstd is much faster than xz, most of the time installing packages is actually spent in fsync(). So you only see huge speedups when run in eatmydata.
Decompressing -21 requires 20MB vs 10MB for -19 and xz.
https://lists.ubuntu.com/archives/ubuntu-devel/2018-March/04...
You can see that the performance gain from zstd under realistic scenarios is about 10%.
you can see that switching from xz to zstd only improved firefox install time from 37s to 33s; running in eatmydata to avoid fsync from dpkg improved it to 12.5s (8.5s with zstd).
I don't know what Arch linux does, but they likely do not correctly fsync() data after its been written to a temporary file, and then fsync the directory after files have been renamed to their final names, as is required to achieve consistent results in a crash.