It is. And it’s not even only Cs fault. 80% of it is bad API Design. Strings could be accepted as a struct consisting of a pointer and a length, aka string_view. And there could be some manipulation functions around it. That would make those APIs a lot more flexible (one no longer needs to care whether things are null terminated and there would be less pointless copies).

For these reasons my estimate in the meantime is that the average C program which uses stdlib functions is less efficient than an implementation in another language, even though the authors would claim otherwise (its C, it must be fast).