"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?"
No, do be afraid of memcpy, as well as malloc. These are two of the four horsemen of poor performance [pl.atyp.us].
After you've picked the best algorithm for your needs, one of the next best optimizations you can do is to avoid unnecessary memory allocation and copying. The key is developing a good eye for "unnecessary." For example, a common anti-pattern in intra-process communication is passing messages by copying one buffer to another. There's usually a better way, one that involves passing a pointer to the message rather than all of its content.
What makes extraneous memory allocation and copying insidious is that no single memory allocation or copy will ruin your program. However, when they're repeated throughout a large code base, the inefficiency adds up--all while becoming a diffuse inefficiency that poses no single bottleneck. That is, once you realize that memory allocation and copying are a problem, it's too late to be an easy fix.