Hrmm, it makes performance claims with regard to Buck1 but not to Bazel, the obvious alternative. Hardly anyone uses Buck1 so you'd think it would be relevant.
I have a non-toy multi-language project in https://github.com/dtolnay/cxx for which I have both Buck2 and Bazel build rules.
On my machine `buck2 clean && time buck2 build :cxx` takes 6.2 seconds.
`bazel clean && time bazel build :cxx` takes 19.9 seconds.