Question: I noticed it some Objective-C projects some of the files that use a significant amount of C functions, store the code in `.c` files instead of `.m`. I know `.m` is for Objective-C and `.c` is for C but is there any benefit to using `.c` over `.m` when you can? Is there a performance increase or complication benefit? Would GCC or clang compile equivalent code differently in each?

Answer: 
The primary benefit is that .c files can be used as-is in normal C programs and you know they won't accidentally incorporate Objective-C constructs that render them incompatible.
Yes, Xcode will compile .c files with the 'C' compiler and the .m files with the 'Objective-C' compiler. It's the same compiler binary, just using different language options. That being the case, I wouldn't expect the relative performance to be too different.