That's being addressed with a new index protocol, specified a while ago[1], available for testing since the middle of the last year[2], and slated for release in a week's time[3].

[1] https://rust-lang.github.io/rfcs/2789-sparse-index.html

[2] https://blog.rust-lang.org/2022/06/22/sparse-registry-testin...

[3] https://blog.rust-lang.org/inside-rust/2023/01/30/cargo-spar...