I'm absolutely on-board ...in application code.

I do feel like it's good, though, when libraries optimize. Ideally, they don't have much else to do than one thing really well anyways.

And with how many libraries modern applications pull in, you do eventually notice whether you're in the Python ecosystem, where most libraries don't care, or in the Rust ecosystem, where many libraries definitely overdo it. Because well, they also kind of don't overdo it, since as a user of the library, you don't see any of it, except the culmulative performance benefits.