> I _think_ Rust is unnecessarily recompiling dependencies on incremental builds? (I don't understand how incremental compilation times can be so bad otherwise). But I'm clueless about how to go about debugging that.

Make sure that your ide/vscode is using the exact same rust toolchain as you to build/check/run. Otherwise you don't get incremental compiling but recompilation a lot. I think there's an environment variable to force it.