__arglist is much slower than params array.

ArgListIsTooSlow

using System;
using System.Diagnostics;

public class ArgListIsTooSlow
{
    static void Main(string[] args)
    {
        const int N = 100000;
        var sw = new Stopwatch();

        sw.Reset();
        sw.Start();
        for (int i = 0; i < N; i++)
        {
            Params.Sum(1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0);
        }
        sw.Stop();
        Console.WriteLine(sw.Elapsed);

        sw.Reset();
        sw.Start();
        for (int i = 0; i < N; i++)
        {
            Arglist.Sum(__arglist(1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0));
        }
        sw.Stop();
        Console.WriteLine(sw.Elapsed);
    }

    class Params
    {
        public static int Sum(params int[] values)
        {
            int sum = 0;
            for (int i = 0; i < values.Length; i++)
            {
                sum += values[i];
            }

            return sum;
        }
    }

    class Arglist
    {
        public static int Sum(__arglist)
        {
            var values = new ArgIterator(__arglist);
            int sum = 0;
            int count = values.GetRemainingCount();
            for (int i = 0; i < count; i++)
            {
                sum += __refvalue(values.GetNextArg(), int);
            }

            return sum;
        }
    }
}