"I've been writing C for quite some time, but I never followed good conventions I'm afraid, and I never payed much attention to the optimization tricks of the higher C programmers. Sure, I use const when I can, I use the pointer methods for manual string copying, I even use register for all the good that does with modern compilers, but now, I'm trying to write a C-string handling library for personal use, but I need speed, and I really don't want to use inline ASM. So, I am wondering, what would other Soylenters do to write efficient, pure, standards-compliant C?"
Any sane string implementation (that is, not C's) will have a length field (or, alternatively, an additional pointer to end), so you don't have to scan the string to find out its length (an O(n) operation which, as you noted, will do bad things to your cache), but can just read the length (or calculate it in O(1) by simple pointer subtraction). This also means you'll not arbitrarily restrict the characters which can be stored in your string (C applications are generally easily identified by them being tripped off by a zero byte in positions supposed to contain text).