Slow code bottlenecked by string operations is probably just bad, though - no matter which way you code your strings. Strings are not what computers like to do. Computers like integers and subscripting operations. Strings can make sense for inter-process scenarios (for example, file paths are usually the right thing and you don't want to reference files with inodes).

There is a culture of bad C code from the 90s - especially C code written in OOP-y ways where that was never necessary. Like, calling malloc() + free() for every little thing instead of more structured memory management. A lot of that code is written in C not with a efficiency or elegance mindset but simply because C was the language that you wrote programs in.