Long - 0.4s NullableLong - 4s Why? Have a look at Joe Albahari's presentation, around 1:02h mark. http://www.youtube.com/watch?v=aZCzG2I8Hds

gistfile1.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Long();

            NullableLong();

            Console.ReadLine();
        }

        private static void Long()
        {
            var stopwatch = Stopwatch.StartNew();
            long result = 0;
            for (long i = 0; i < 100000000; i++)
            {
                result = result + i;
            }

            stopwatch.Stop();

            Console.WriteLine(stopwatch.Elapsed);
        }

        private static void NullableLong()
        {
            var stopwatch = Stopwatch.StartNew();
            long? result = 0;
            for (long? i = 0; i < 100000000; i++)
            {
                result = result + i;
            }

            stopwatch.Stop();

            Console.WriteLine(stopwatch.Elapsed);
        }
    }
}

Yep, Joe Albahari explains it nicely