Thanks for that very helpful tip! According to the output of that, no unnecessary recompilation is taking place.

I actually figured out the issue! It turns out, I had unquestioningly transplanted the following from the blog post in discussion, and that is the cause of all my issues:

    [profile.dev]
    opt-level = 1

    [profile.dev.package."*"]
    opt-level = 3

Removing that seems to give me an order (or two) of magnitude reduction in compile times.

(Worth noting: it was passing RUSTFLAGS=-Ztime-passes that made me look here because, apparently, `llvm_passes` was taking the majority of the time).