So how do you write programs without data structures? You copy paste some implementation and modify it slightly at every use site?

Possible optimizations depend on the semantics of the language — C++ can do just orders of magnitude more with for example a string data structure, than what is possible with a C “string”. So no, C is not the most performant language in general — de facto C++ is used for everything performance critical.

> You copy paste some implementation and modify it slightly at every use site?

Yes, or use macros. Not saying it's the most convenient, but that's an orthogonal concern.

You wouldn't say hand-written assembly isn't performant, so your objection strikes me as wrong.