连接两个List<用foreach循环

我试图稍微远离LINQ，它被证明是非常有用的，但有时也很难阅读。

我曾经使用LINQ来执行连接(完整的外连接)，但为了简单起见，我更喜欢使用for/foreach循环。我只是将一条LINQ语句(不是PLINQ)转换为嵌套的foreach循环，性能受到严重影响。过去需要几秒钟的工作现在大约需要一分钟，见下面的代码。

foreach (var p in PortfolioELT)
{
    double meanloss;
    double expvalue;
    double stddevc;
    double stddevi;
    bool matched = false;
    foreach (var a in AccountELT)
    {
        if (a.eventid == p.eventid)
        { DO SOME MATH HERE <-----

有什么想法吗

1. 为什么比LINQ Join和
慢?
2. 我怎样才能加快速度?

程序很明显地做了它需要做的事情，但是太慢了。

编辑:旧代码FULL

public static ConcurrentList<Event> CreateNewELTSUB(IList<Event> AccountELT, IList<Event> PortfolioELT)
    {
        if (AccountELT == null)
        {
            return (ConcurrentList<Event>)PortfolioELT;
        }
        else
        {
            //Subtract the Account ELT from the Portfolio ELT
            var newELT = from p in PortfolioELT
                         join a in AccountELT
                         on p.eventid equals a.eventid into g
                         from e in g.DefaultIfEmpty()
                         select new
                         {
                             EventID = p.eventid,
                             Rate = p.rate,
                             meanloss = p.meanloss - (e == null ? 0d : e.meanloss),
                             expValue = p.expValue - (e == null ? 0d : e.expValue),
                             stddevc = Math.Sqrt(Math.Pow(p.stddevc, 2) - (e == null ? 0d : Math.Pow(e.stddevc, 2))),
                             stddevi = Math.Sqrt(Math.Pow(p.stddevi, 2) - (e == null ? 0d : Math.Pow(e.stddevi, 2)))
                         };

            ConcurrentList<Event> list = new ConcurrentList<Event>();
            foreach (var x in newELT)
            {
                list.Add(new Event(x.meanloss, x.EventID, x.expValue, x.Rate, x.stddevc, x.stddevi));
            }
            return list;
        }
    }

NEW CODE FULL:

    public static ConcurrentList<Event> CreateNewELTSUB(IList<Event> AccountELT, IList<Event> PortfolioELT)
    {
        if (AccountELT == null)
        {
            return (ConcurrentList<Event>)PortfolioELT;
        }
        else
        {
            //Subtract the Account ELT from the Portfolio ELT
            ConcurrentList<Event> newlist = new ConcurrentList<Event>();
            //Outer Join on Portfolio ELT
            foreach (var p in PortfolioELT)
            {
                double meanloss;
                double expvalue;
                double stddevc;
                double stddevi;
                bool matched = false;
                foreach (var a in AccountELT)
                {
                    if (a.eventid == p.eventid)
                    {
                        matched = true;
                        meanloss = p.meanloss - a.meanloss;
                        expvalue = p.expValue - a.expValue;
                        stddevc = Math.Sqrt((Math.Pow(p.stddevc, 2)) - (Math.Pow(a.stddevc, 2)));
                        stddevi = Math.Sqrt((Math.Pow(p.stddevi, 2)) - (Math.Pow(a.stddevi, 2)));
                        newlist.Add(new Event(meanloss, p.eventid, expvalue, p.rate, stddevc, stddevi));
                    }
                    else if (a.eventid != p.eventid)    //Outer Join on Account
                    {
                        newlist.Add(a);
                    }
                }
                if (!matched)
                {
                    newlist.Add(p);
                }
            }
            return newlist;
        }