I have tried setting the build dependencies optimization level to higher-than-default in the past, but it seems to achieve nothing.

    [profile.dev.build-override]
    opt-level = 3
    codegen-units = 1
    
    [profile.dev.package."syn"]
    opt-level = 3
    codegen-units = 1
    
    [profile.dev.package."synstructure"]
    opt-level = 3
    codegen-units = 1
    
    [profile.dev.package."quote"]
    opt-level = 3
    codegen-units = 1

    # CTRL+C when it reaches async-stripe, then repeat
    > rm -r target/debug/build/async-stripe*; time cargo build

default, O1, O2, O3:

    Executed in   47.13 secs
    Executed in   45.68 secs
    Executed in   46.07 secs
    Executed in   46.22 secs

sccache will provide a major benefit, especially in combination with `cargo-hakari`.