> And the borrow checker is SLOW, and unavoidably so

IIRC for most codebases borrowck actually makes up a pretty small proportion of compilation time (e.g., see [0] for some admittedly rough estimates).

> but borrow checking could have been avoided with different lifetime rules.

I'm not sure you could entirely avoid borrow checking without a pretty significant change to what you're allowed to do with references.

[0]: https://kobzol.github.io/rust/rustc/2024/03/15/rustc-what-ta...