I’ve specifically used Opus to diagnose and fix performance bottlenecks in parallel Rust code on multiple occasions (e.g improving NPS for a chess engine) and it works well.

I’ve done plenty of performance architecting in my day-job and rule #1 is generally “you can’t fix what you can’t see/measure”. I have a suspicion that many folks aren’t investing in letting AI actually introspect iterative execution via the appropriate harness, and are then acting surprised that it is no oracle.