What LLMs are you using? Just simply telling Sol 6.1, Opus 5.5, Fable or Astra to performance test something will get you a solid improvement in poorly optimized code.

Giving it a specific plan will get you a solid test harness.

And setting up an autoresearch system and running it overnight will get you expert level optimizations if you set the metric up right.

These things excel at performance optimizations.