thanks for the tips. As my data has very low entropy, both can compress down to 3-4% of original size, but xz is a lot faster in compression.

raw size: 9612344 B

zstd --ultra -22 --long=31 => 376181 B (3.91% original, 4.088s compress, 0.013s decompress)

xz -z -9 xml => 353700 B (3.68% original, 0.729s compress, 0.032s decompress)

zstd -17 --long=31 could match the compression time of xz, but the size is bigger (405602 B, 4.22% original)

If you compare only the compressed size (not to the original size), .zst would be about 6-15% larger than .xz