Rust 1.24 now sets codegen-units to 16 by default to speed up compilation time but it makes the final binary slower https://blog.rust-lang.org/2018/02/15/Rust-1.24 > For maximum speed, setting `codegen-units` to `1` in your `Cargo.toml` is needed to eke out every last drop of performance. So [src/rust/Cargo.toml](https://gitweb.torproject.org/tor.git/tree/src/rust/Cargo.toml) should be changed with that to squeeze the most perf.