You presented another thing LLMs excel at: integrating something from a project that is not present in another one. I think they work so well at this because both the starting and ending points have an already existing structure, so the LLM can guide itself effectively. In your case it's even more egregious because we are talking about the same exact algorithm/functionality implemented in two different, but rather similar, programming languages.

Could you have manually profiled and compared the execution paths? Sure. Could you have translated the C++ optimizations to C#? Sure. But in such an obvious case, the LLM managed itself.