Why is Calli Faster Than a Delegate Call?

I was playing around with Reflection.Emit and found about about the little-used EmitCalli. Intrigued, I wondered if it's any different from a regular method call, so I whipped up the code below:

using System;
using System.Diagnostics;
using System.Reflection.Emit;
using System.Runtime.InteropServices;
using System.Security;

[SuppressUnmanagedCodeSecurity]
static class Program
{
    const long COUNT = 1 << 22;
    static readonly byte[] multiply = IntPtr.Size == sizeof(int) ?
      new byte[] { 0x8B, 0x44, 0x24, 0x04, 0x0F, 0xAF, 0x44, 0x24, 0x08, 0xC3 }
    : new byte[] { 0x0f, 0xaf, 0xca, 0x8b, 0xc1, 0xc3 };

    static void Main()
    {
        var handle = GCHandle.Alloc(multiply, GCHandleType.Pinned);
        try
        {
            //Make the native method executable
            uint old;
            VirtualProtect(handle.AddrOfPinnedObject(),
                (IntPtr)multiply.Length, 0x40, out old);
            var mulDelegate = (BinaryOp)Marshal.GetDelegateForFunctionPointer(
                handle.AddrOfPinnedObject(), typeof(BinaryOp));

            var T = typeof(uint); //To avoid redundant typing

            //Generate the method
            var method = new DynamicMethod("Mul", T,
                new Type[] { T, T }, T.Module);
            var gen = method.GetILGenerator();
            gen.Emit(OpCodes.Ldarg_0);
            gen.Emit(OpCodes.Ldarg_1);
            gen.Emit(OpCodes.Ldc_I8, (long)handle.AddrOfPinnedObject());
            gen.Emit(OpCodes.Conv_I);
            gen.EmitCalli(OpCodes.Calli, CallingConvention.StdCall,
                T, new Type[] { T, T });
            gen.Emit(OpCodes.Ret);

            var mulCalli = (BinaryOp)method.CreateDelegate(typeof(BinaryOp));

            var sw = Stopwatch.StartNew();
            for (int i = 0; i < COUNT; i++) { mulDelegate(2, 3); }
            Console.WriteLine("Delegate: {0:N0}", sw.ElapsedMilliseconds);
            sw.Reset();

            sw.Start();
            for (int i = 0; i < COUNT; i++) { mulCalli(2, 3); }
            Console.WriteLine("Calli:    {0:N0}", sw.ElapsedMilliseconds);
        }
        finally { handle.Free(); }
    }

    delegate uint BinaryOp(uint a, uint b);

    [DllImport("kernel32.dll", SetLastError = true)]
    static extern bool VirtualProtect(
        IntPtr address, IntPtr size, uint protect, out uint oldProtect);
}

I ran the code in x86 mode and x64 mode. The results?

32-bit:

• Delegate version: 994
• Calli version: 46

64-bit:

• Delegate version: 326
• Calli version: 83

I guess the question's obvious by now... why is there such a huge speed difference?

Update:

I created a 64-bit P/Invoke version as well:

• Delegate version: 284
• Calli version: 77
• P/Invoke version: 31

Apparently, P/Invoke is faster... is this a problem with my benchmarking, or is there something going on I don't understand? (I'm in release mode, by the way.)