I tried a few times with zstd at various levels of compression with the linux kernel sources. While I've been impressed with zstd, and have some projects lined up to use it, it seems in the case of the linux kernel sources, it's not a great fit. xz handily beats it, and not by a small margin either. I had to really ratchet up the compression levels (20+) before I could get close to 100Mb.
In general, xz beats zstd in compression ratio, as xz is very committed to providing the strongest compression, at the expense of speed, while zstd provides a range of compression ratio vs speed tradeoffs [0]. At the lower levels, zstd isn't approaching xz's compression level, but it's doing it much much faster. Additionally, zstd generally massively outperforms xz in decompression speed
$ time xz linux-4.14-rc6.tar
real 4m26.009s
user 4m24.828s
sys 0m0.724s
$ wc -c linux-4.14-rc6.tar.xz
103705148 linux-4.14-rc6.tar.xz
$ time zstd --ultra -20 linux-4.14-rc6.tar
linux-4.14-rc6.tar : 12.81% (824350720 => 105564246 bytes, linux-4.14-rc6.tar.zst)
real 4m34.129s
user 4m33.484s
sys 0m0.432s
$ time cat linux-4.14-rc6.tar.xz | xz -d > out1
real 0m9.677s
user 0m6.608s
sys 0m0.704s
$ time cat linux-4.14-rc6.tar.zst | zstd -d > out2
real 0m1.702s
user 0m1.220s
sys 0m0.520s
