>I've tried asking Claude to optimize it further, it created a plan that looks reasonable (I've never interacted with Rust in my life) and it spent a day building many of these optimizations but at the end of the day, none of them actually improved the runtime and some even made it way worse.

This is the kind of thing where if this was a real developer tweaking a codebase they're familiar with, it could get done, but with AI there's a glass ceiling