I too fixed performance problems in that repo a few years back and did a write up on it - https://jackson.dev/post/rust-coreutils-dd/
I'm glad this project is getting some more attention, maybe even getting funding from Ubuntu since they're using it? Last time I touched it most of the code was still pretty clearly written by Rust beginners and non-systems programmers so it likely had/has many such issues to uncover. Ubuntu putting it into their distro should hopefully get more experienced (and actually paid!) devs taking a closer look.
I fully agree that nixos moving off GitHub is a nonstarter for technical reasons. The nixpkgs repo is enormous and has caused loads of infrastructure problems at GitHub over the years.
But the NixOS foundation currently has over 300K Euros in their open collective account. They get free sponsorship from GitHub for the repo and free sponsorships covering the very expensive build and caching services too.
https://opencollective.com/nixos