This week I had to deal with the exact thing you mention. A pull request for "performance optimization", basically a proposal to unwrap a higher abstraction to do it at a lower level, manually and directly.

Yes, it does save a few function calls and instantiating a class - but that's not where the real bottleneck is. The proposed changes would require rewriting large amounts of code elsewhere, making the logic more verbose and complex.

It took some effort to convince the person that the trade-off isn't worth it. I tried to be diplomatic, but I'm afraid they took it personally that the pull request was declined.

I wish I could have proved my objection with some metrics - how many milliseconds were shaved off by the proposal, versus the increased "cyclomatic complexity", as a measurement of its detrimental effect on the code base.