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: 
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.
Besides everything else, .c is good for cross platform development. The overhead of porting it from one platform to another is very low or almost negligible. . And mostly the backend or the model of an application is developed using "C" where performance is required and you need a direct interaction with the machine. Such as interaction with hardware and stuff.