Why is strcmp so much faster than my function?

I wrote a function, Str::Compare, that is basically a strcmp rewritten in another way. While comparing the two function, in a loop repeted 500'000'000 times, strcmp execute too much fast, about x750 times faster.

This code was compiled in a C library with -Os parameter active:

int Str::Compare(char* String_1, char* String_2)
{
    char TempChar_1, TempChar_2;

   do
   {
        TempChar_1 = *String_1++;
        TempChar_2 = *String_2++;
   } while(TempChar_1 && TempChar_1 == TempChar_2);

   return TempChar_1 - TempChar_2;
}

The execution time of that function is 3.058s, while strcmp only 0.004s.

Why this happen?

Also this is how I implemented the benchmark loop:

int main()
{
     char Xx[] = {"huehuehuehuehuehuehuehuehuehuehuehuehuehue"},
          Yy[] = {"huehuehuehuehuehuehuehuehuehuehuehuehuehue"};
     for(int i = 0; i < 500000000; ++i)
         Str::Compare(Xx, Yy);
}

Edit: While testing some code I wrote and optimization that improved drastically Str::Compare speed. If before strcmp was x750 times faster now is only x250. This is the new code:

int Str::Compare(char* String_1, char* String_2)
{
     char TempChar_1, TempChar_2, TempChar_3;

     while(TempChar_1 && !TempChar_3)
     {
          TempChar_1 = *String_1++;
          TempChar_2 = *String_2++;
          TempChar_3 = TempChar_1 ^ TempChar_2;
     }

     return TempChar_1 - TempChar_2;
}

New execution time is 0.994s.