Except when the program is actually written in C, then better hold the Algorithms and Data Structures book and dust it off, or Intel/AMD/ARM/... manuals.
Algorithms and data structures come BEFORE dropping to c.
These days it is rare that you can beat your compiler with hand machine code, and even if you can it isn't worth it because the difference is typically small and only applies to one specific machine.
Of course once in C you can often think about memory locality and other cache factors that higher languages hide from you.
