KMP algorithm challenge string.Contains

KMP:

 public int KMP (ReadOnlySpan<char> content, ReadOnlySpan<char> span) {
            _next = new int[span.Length];
            GetNext (span);
            int i = 0;
            int j = 0;
            while (i < content.Length && j < span.Length) {
                if (j == 0 || content[i] == span[j]) {
                     if(content[i]==span[j])
                        j++;
                    i++;

                } else
                    j = _next[j];
            }
            if (j >= span.Length)
                return i - span.Length;
            else
                return -1;

        }

        private void GetNext (ReadOnlySpan<char> content) {
            _next[0] = 0;
            for (int i = 1; i < _next.Length; i++) {
                if (_next[i - 1] != 0) {
                    if (content[i] == content[_next[i - 1]])
                        _next[i] = _next[i - 1] + 1;
                } else {
                    if (content[0] == content[i])
                        _next[i] = 1;
                    else
                        _next[i] = 0;
                }
            }
        }

string.Contains and KMP benchmarks:

  [Benchmark]
        public void TestStringContain()
        {
            string s = "abcasdfasfdkjefasdfasdaaadfdfasdfasdfasdfjjsdjfjsglskdfjskdjfaskjdflkasjgksajdfksjdf";
            bool x = s.Contains("asdfasd",StringComparison.Ordinal);
        }

         [Benchmark]
        public void TestKMP()
        {
            int x = containKeyWords.KMP("abcasdfasfdkjefasdfasdaaadfdfasdfasdfasdfjjsdjfjsglskdfjskdjfaskjdflkasjgksajdfksjdf", "asdfasd");
        }

result:

Complete defeat!