But they not stated it is <because> of C#'s performance, so I don't think this is THAT problematic. But I agree that it would be fine to see them dogfeeding on their language for such a massive project, and a project that is even related to TypeScript (as it inspired it in some features), it is a shame they don't do it, but it is also the case for many of their projects (like, they are even pushing react native for apps nowadays), so I think at some level it's really fine.