You have to be clever sometimes, take strlen in C you could do byte at a time which a compiler will vectorize or you could do SWAR with some fancy bit hacks and get a large perf boost.
A better statement might be: "Don't be clever, let others be clever for you."
Obviously, stdlib functions are going to be highly optimized and the compiler will put hardware-specific performance hacks in for your benefit. Smarter people than you wrote, vetted, and approved that code. Use it, and don't ever try to be that smart yourself.
