Parallel.ForEach与foreach相比的优势怎么体现[通俗易懂]

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

namespace ConsoleApplication1
{
    public class People
    {
        private int id;

        public int Id
        {
            get { return id; }
            set { id = value; }
        }
        private string name;

        public string Name
        {
            get { return name; }
            set { name = value; }
        }
        private int age;

        public int Age
        {
            get { return age; }
            set { age = value; }
        }
        private Byte sex;

        public Byte Sex
        {
            get { return sex; }
            set { sex = value; }
        }
        private string school;

        public string School
        {
            get { return school; }
            set { school = value; }
        }

    }
    class Program
    {
        static void Main(string[] args)
        {
            List<People> pd = new List<People>();
            for(int i = 0; i < 72000; i++)
            {
                People people = new People();
                people.Id = i;
                people.Sex = 1;
                if(i<36000){
                    people.Age = 23;
                    people.Name= "MJ" + i;
                    people.School="NC";
                }else
                {
                    people.Name= "KD" + i;
                    people.Age = 24;
                    people.School="UCLA";
                }
                pd.Add(people);
            }

            System.DateTime currentTime0 = System.DateTime.Now;
            foreach (People p in pd)
            {
                p.Id += 1;
                p.Name = "aaa";
                p.Age = 40;
                p.Sex = 1;
                Console.WriteLine(p.Id + ", " + p.Name + ", " + p.School);
            }
            System.DateTime currentTime1 = System.DateTime.Now;
            Parallel.ForEach(pd, p =>
            {
                p.Id += 1;
                p.Name = "bbb";
                p.Age = 35;
                p.Sex =0;
                Console.WriteLine(p.Id + ", " + p.Name + ", " + p.School);
            });
            System.DateTime currentTime2 = System.DateTime.Now;

            Console.WriteLine(currentTime0.ToString() + " " + currentTime0.Millisecond.ToString());
            Console.WriteLine(currentTime1.ToString() + " " + currentTime1.Millisecond.ToString());
            Console.WriteLine(currentTime2.ToString() + " " + currentTime2.Millisecond.ToString());
            Console.ReadKey();
        }
    }
}

输出过程中，明显看到第二个循环比第一个快了很多很多，但是看最后的时间差却失望了：

2014/08/29 15:37:50 401

2014/08/29 15:37:59 246

2014/08/29 15:38:08 185