Reto MSDN 7: http://blogs.msdn.com/b/esmsdn/archive/2014/11/28/retosmsdn-reto-7-procesando-json-en-c.aspx

1_JsonSplitterLazy.cs

using Newtonsoft.Json;
using Newtonsoft.Json.Linq;
using System.Linq;

// http://json.codeplex.com/
// NUGET: install-package Newtonsoft.Json
// MY TIME: Millisecs = 0, Ticks = 588  (total 357 ms)

namespace Reto7
{
    public class JsonSplitter
    {
        public static JsonSplitter SplitShowsByGenre(string json, string genre)  {
            return new JsonSplitter { Json = json, Genre = genre };
        }

        public string Json { get; private set; }
        public string Genre { get; private set; }

        public string Item1 {
            get {
                return JsonConvert.SerializeObject((JsonConvert.DeserializeObject(Json) as JArray).
                    Where(x => x.SelectToken("genres").Values<string>().Contains(Genre)), Formatting.None);
            }
        }

        public string Item2 {
            get {
                return JsonConvert.SerializeObject((JsonConvert.DeserializeObject(Json) as JArray).
                     Where(x => !x.SelectToken("genres").Values<string>().Contains(Genre)), Formatting.None);
            }
        }
    }
}

2_JsonSplitterNormal.cs

using Newtonsoft.Json;
using Newtonsoft.Json.Linq;
using System.Linq;

// http://json.codeplex.com/
// NUGET: install-package Newtonsoft.Json
// MY TIME: Millisecs = 226, Ticks = 810927  (total 237 ms)

namespace Reto7
{
    public class JsonSplitter
    {
        public static JsonSplitter SplitShowsByGenre(string json, string genre)
        {
            var gr = (JsonConvert.DeserializeObject(json) as JArray).GroupBy
                (x => x.SelectToken("genres").Values<string>().Contains(genre));
            return new JsonSplitter
            {
                Item1 = JsonConvert.SerializeObject(gr.Where(x => x.Key).First(), Formatting.None),
                Item2 = JsonConvert.SerializeObject(gr.Where(x => !x.Key).First(), Formatting.None),
            };
        }

        public string Item1 { get; private set; }
        public string Item2 { get; private set; }

    }
}

3_JsonSplitterFast.cs

using System.Text;
using System.Text.RegularExpressions;
using System.Globalization;

// MY TIME: Millisecs = 61, Ticks = 220120  (total 72 ms)

namespace Reto7
{
    public class JsonSplitter
    {
        public static JsonSplitter SplitShowsByGenre(string json, string genre)
        {
            json = DecodeUnicodeChars(json);
            var find = '"' + genre + '"';
            var str1 = new StringBuilder();
            var str2 = new StringBuilder();
            int firstPosShow = -1, lastPosShow = -1;
            while (json[lastPosShow + 1] != ']') {
                firstPosShow = lastPosShow + 2; // skip [{ or ,{
                lastPosShow = json.IndexOf('}', json.IndexOf('}', json.IndexOf('}', lastPosShow + 1) + 1) + 1); // skip images, rating, show itself
                var genreIni = json.IndexOf('[', firstPosShow + 1); // genre is the only array
                var genreIndex = json.IndexOf(find, genreIni + 1);
                var choose = (genreIndex != -1 && genreIndex < json.IndexOf(']', genreIni + 1)) ? str1 : str2;
                choose.Append(choose.Length == 0 ? '[' : ',');
                choose.Append(json, firstPosShow, lastPosShow - firstPosShow + 1);
            }
            return new JsonSplitter {
                Item1 = str1.Append(']').ToString(),
                Item2 = str2.Append(']').ToString()
            };
        }

        private static Regex _regex = new Regex(@"\\u(?<Value>[a-zA-Z0-9]{4})", RegexOptions.Compiled);
        public static string DecodeUnicodeChars(string value) {
            return _regex.Replace(
                value,
                m => ((char)int.Parse(m.Groups["Value"].Value, NumberStyles.HexNumber)).ToString()
            );
        }

        public string Item1 { get; private set; }
        public string Item2 { get; private set; }

    }
}

4_JsonSplitterFast2.cs

using System.Text;

namespace Reto7
{
    public class JsonSplitter
    {
        public static JsonSplitter SplitShowsByGenre(string json, string genre) {
            json = DecodeUnicodeChars(json);
  
            var find = '"' + genre + '"';
            var str1 = new StringBuilder();
            var str2 = new StringBuilder();
            int firstPosShow = -1, lastPosShow = -1;
            while (json[lastPosShow + 1] != ']') {
                firstPosShow = lastPosShow + 2; // skip ,{
                lastPosShow = json.IndexOf('}', json.IndexOf('}', json.IndexOf('}', lastPosShow + 1) + 1) + 1); // skip images, rating, show itself
                var genreIni = json.IndexOf('[', firstPosShow + 1); // genre is the only array
                var genreIndex = json.IndexOf(find, genreIni + 1);
                var choose = (genreIndex != -1 && genreIndex < json.IndexOf(']', genreIni + 1)) ? str1 : str2;
                choose.Append(choose.Length == 0 ? '[' : ',');
                choose.Append(json, firstPosShow, lastPosShow - firstPosShow + 1);
            }
            return new JsonSplitter {
                Item1 = str1.Append(']').ToString(),
                Item2 = str2.Append(']').ToString()
            };
        }

        private static string DecodeUnicodeChars(string str) {
            var ret = new StringBuilder();
            int pos = -1, lastPos = 0;
            while ( (pos = str.IndexOf("\\u", lastPos)) != -1) {
                ret.Append(str, lastPos, pos - lastPos); // don't include \
                ret.Append((char)(HexNumber(str[pos + 2]) << 12 | HexNumber(str[pos + 3]) << 8 |
                    HexNumber(str[pos + 4]) << 4 | HexNumber(str[pos + 5])));
                lastPos = pos + 6;
            }
            return ret.Append(str, lastPos, str.Length - lastPos ).ToString();
        }

        private static int HexNumber(char letter) {
            return (letter <= '9') ? letter - '0' : letter - 'a' + 10;
        }

        public string Item1 { get; private set; }
        public string Item2 { get; private set; }
    }
}