c# - For vs Foreach -巨大的性能差异

我正在对一个算法进行一些优化，该算法可以在给定数组中找到比X大的最小数字，但是后来我偶然发现了一个奇怪的差异。在下面的代码中，"ForeachUpper"结束于625ms，而"ForUpper"结束于，我相信，几个小时(慢得离谱)。为什么如此?

  class Teste
{
    public double Valor { get; set; }
    public Teste(double d)
    {
        Valor = d;
    }
    public override string ToString()
    {
        return "Teste: " + Valor;
    }
}
  private static IEnumerable<Teste> GetTeste(double total)
    {
        for (int i = 1; i <= total; i++)
        {
            yield return new Teste(i);
        }
    }
    static void Main(string[] args)
    {
        int total = 1000 * 1000*30 ;
        double test = total/2+.7;
        var ieTeste = GetTeste(total).ToList();

        Console.WriteLine("------------");
        ForeachUpper(ieTeste.Select(d=>d.Valor), test);
        Console.WriteLine("------------");
        ForUpper(ieTeste.Select(d => d.Valor), test);
        Console.Read();
    }
    private static void ForUpper(IEnumerable<double> bigList, double find)
    {
        var start1 = DateTime.Now;
        double uppper = 0;
        for (int i = 0; i < bigList.Count(); i++)
        {
            var toMatch = bigList.ElementAt(i);
            if (toMatch >= find)
            {
                uppper = toMatch;
                break;
            }
        }
        var end1 = (DateTime.Now - start1).TotalMilliseconds;
        Console.WriteLine(end1 + " = " + uppper);
    }
    private static void ForeachUpper(IEnumerable<double> bigList, double find)
    {
        var start1 = DateTime.Now;
        double upper = 0;
        foreach (var toMatch in bigList)
        {
            if (toMatch >= find)
            {
                upper = toMatch;
                break;
            }
        }
        var end1 = (DateTime.Now - start1).TotalMilliseconds;
        Console.WriteLine(end1 + " = " + upper);
    }

谢谢