Позволю себе продолжить ваши изыскания:
json.codeplex.com/releases/view/64935
The test of binary formatter:
1000 iterations in 122 ms
2000 iterations in 186 ms
3000 iterations in 286 ms
4000 iterations in 358 ms
5000 iterations in 450 ms

The test of protobuf-net:
1000 iterations in 137 ms
2000 iterations in 47 ms
3000 iterations in 72 ms
4000 iterations in 93 ms
5000 iterations in 118 ms

The test of json-net:
1000 iterations in 232 ms
2000 iterations in 200 ms
3000 iterations in 313 ms
4000 iterations in 406 ms
5000 iterations in 513 ms

The comparision of file size:
The size of tasks1.bin is 725 bytes
The size of tasks2.bin is 101 bytes
The size of tasks3.bin is 244 bytes

private static void TestJson(IList tasks, string fileName, int iterationCount)
{
var stopwatch = new Stopwatch();

using (var file = File.Create(fileName))
{
stopwatch.Restart();
for (var i = 0; i < iterationCount; i++)
{
var str = JsonConvert.SerializeObject(tasks);

var bytes = Encoding.UTF8.GetBytes(str);
file.Write(bytes, 0, bytes.Length);
file.Read(bytes, 0, bytes.Length);
file.Position = 0;
var restoredTasks = JsonConvert.DeserializeObject(Encoding.UTF8.GetString(bytes));
}

stopwatch.Stop();

Console.WriteLine("{0} iterations in {1} ms", iterationCount, stopwatch.ElapsedMilliseconds);
}
}