Why are standard string functions faster than my custom string functions?

I decided to find the speeds of 2 functions :

• strcmp - The standard comparison function defined in string.h
• xstrcmp- A function that has same parameters and does the same, just that I created it.

Here is my xstrcmp function :

int xstrlen(char *str)
{
    int i;
    for(i=0;;i++)
    {
        if(str[i]=='\0')
            break;
    }
    return i;
}

int xstrcmp(char *str1, char *str2)
{
    int i, k;
    if(xstrlen(str1)!=xstrlen(str2))
        return -1;
    k=xstrlen(str1)-1;
    for(i=0;i<=k;i++)
    {
        if(str1[i]!=str2[i])
            return -1;
    }
    return 0;
}

I didn't want to depend on strlen, since I want everything user-defined.

So, I found the results. strcmp did 364 comparisons per millisecond and my xstrcmp did just 20 comparisons per millisecond (atleast on my computer!)

Can anyone tell why this is so ? What does the xstrcmp function do to make itself so fast ?