Why is memcmp so much faster than a for loop check?

Question

Why is memcmp(a, b, size) so much faster than:

for(i = 0; i < nelements; i++) {
    if a[i] != b[i] return 0;
}
return 1;

I

Answer 1

Is memcmp a CPU instruction or something?

It is at least a very highly optimized compiler-provided intrinsic function. Possibly a single machine instruction, or two, depending on the platform, which you haven't specified.