Why is there such a large difference in the performance of different ways to pass delegates?

I was attempting to compare three different ways of passing a delegate to a function in C# -- by lambda, by delegate, and by direct reference. What really surprised me was the direct reference method (i.e. ComputeStringFunctionViaFunc(object[i].ToString)) was six times slower than the other methods. Does anyone know why this is?

The complete code is as below:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.CompilerServices;

namespace FunctionInvocationTest
{
    class Program
    {
        static void Main(string[] args)
        {
            object[] objectArray = new object[10000000];
            for (int i = 0; i < objectArray.Length; ++i) { objectArray[i] = new object(); }

            ComputeStringFunction(objectArray[0]);
            ComputeStringFunctionViaFunc(objectArray[0].ToString);
            ComputeStringFunctionViaFunc(delegate() { return objectArray[0].ToString(); });
            ComputeStringFunctionViaFunc(() => objectArray[0].ToString());

            System.Diagnostics.Stopwatch s = new System.Diagnostics.Stopwatch();
            s.Start();
            for (int i = 0; i < objectArray.Length; ++i)
            {
                ComputeStringFunction(objectArray[i]);
            }
            s.Stop();
            Console.WriteLine(s.Elapsed.TotalMilliseconds);

            s.Reset();
            s.Start();
            for (int i = 0; i < objectArray.Length; ++i)
            {
                ComputeStringFunctionViaFunc(delegate() { return objectArray[i].ToString(); });
            }
            s.Stop();
            Console.WriteLine(s.Elapsed.TotalMilliseconds);

            s.Reset();
            s.Start();
            for (int i = 0; i < objectArray.Length; ++i)
            {
                ComputeStringFunctionViaFunc(objectArray[i].ToString);
            }
            s.Stop();
            Console.WriteLine(s.Elapsed.TotalMill