> You can get decent improvements by just switching deflate for zstd

Maybe I sounded too critical of zstd. To be clear: I use it for general-purpose compression where available, the only exception would be where eeking out the last % gain is important and slow decompression is acceptable and it has one of these patterns that Bzip2 does better in the first place

That it's better than deflate (afaik aka gzip and zlib, just with different header fields) is not surprising since that was iirc the defined goal of Zstandard project