为什么Parallel.ForEach比顺序循环慢两倍

为什么下面的indexed parallel.foreach与正常的顺序循环相比几乎慢两倍？它与资源争用有关吗？

static void Main(string[] args)
{
    var values = Enumerable.Range(0, 100000).Select(v => v+v);
    Stopwatch s = Stopwatch.StartNew();
    //int index = 0;
    //foreach (double value in values)
    //{
    //    Console.WriteLine("{0}:'t{1}", index, value);
    //    index++;
    //}
    Parallel.ForEach(values, (value, pls, index) =>
    {
        Console.WriteLine("{0}:'t{1}", index, value);
        index++;
    });
    Console.WriteLine(s.Elapsed.TotalMilliseconds);
    Console.ReadLine();
}