"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?"
Main thing about "don't be clever" is to use OTHER CLEVERER people's code (and benchmark).
There are other C string libraries out there, why not use those? Why are you rolling your own? If you said you wanted to learn about stuff then rolling your own makes sense, BUT you said you needed speed.
Less code to write, fewer bugs you're responsible for and less documentation to do.