I had the exact experience yesterday.

I have a performance problem and went down the path of optimising part of a pipeline that when benchmarked was not the bottleneck, even if it looked plausible for me and the llm. When I asked it to make a final benchmark for documentation I found most of the work I did improved 30% while another path would have improved a magnitude more.

Thankfully iteration is now faster than ever and given how fast it creates tests, previous tests created for the aborted optimisation were helpful.