C is often cited as being a super fast low-level language. There are a few caveats however, and you should know all of them. This collection is written in the hope that you would be able to write better C code after reading this.
P.S. I personally prefer Rust over C/C++.
What Every C Programmer Should Know About Undefined Behavior
Author: LLVM authors (Chris Lattner)
Type: Blog series
Description: The best writeup on undefined behavior so far
Tags: Undefined behavior, Performance
Link: What Every C Programmer Should Know About Undefined Behavior
Low Level Bit Hacks You Absolutely Must Know
Author: Peteris Krumins
Type: Blog post
Description: Very useful list of bit twiddling hacks, explained very well.
Tags: Bit twiddling, Performance
Link: Low Level Bit Hacks You Absolutely Must Know