They compare pretty closely. The squash benchmark is a nice interactive comparison tool that usually agrees with my own benchmarks. See for example [1]. (Note that you should experiment with different input texts to get a sense for the variability in relative performance.)

I am working on that very question! Zstd's support for creating and using custom dictionaries opens the door to significant efficiencies. As described in the post though, dictionaries make compression a more complicated thing to use, and so there are lots of questions about how to apply that to the public internet in a way that's cross-compatible and secure. In short: it's something we're actively exploring.

[1] http://quixdb.github.io/squash-benchmark/unstable/?dataset=e...