我的代码启动速度非常快，但速度却非常慢

我下面的代码运行良好，我根本没有任何异常。它将以极快的速度运行大约 100 秒，然后减慢到爬行。我遵循了某人关于相关问题的建议，并在 Vs 中暂停了我的程序大约 10 次，当它变慢并且它一直击中我将在下面标记的同一行时。

我唯一的理论是我使用的是parallel foreach循环，我相信它就像一个多线程应用程序，它在完成之前继续代码，并且它正在点击循环之后的代码，这是它不断变慢的地方。

为了让您理解下面的代码，表和适配器都是我在代码中访问的强类型 sql 数据集的一部分。我不明白速度问题与并行 foreach 循环有什么关系......

public static void startDownload()
    {
                List<string> markets = new List<string>() { "amex", "global", "nasdaq", "nyse" };
                Parallel.ForEach(markets, market =>
                {
                    IEnumerable<string> symbolList = Helper.getStockSymbols(market);
                    var historicalGroups = symbolList.Select((x, i) => new { x, i })
                      .GroupBy(x => x.i / 100)
                      .Select(g => g.Select(x => x.x).ToArray());
                    historicalGroups.AsParallel().ForAll(g => Downloads.getHistoricalStockData(g, market));
                });
            }
            // wait 24 hours before you do this again
            Task.Delay(TimeSpan.FromHours(24)).Wait();
    }
public static List<string> getStockSymbols(string market)
    {
        List<string> symbolList = new List<string>();
            using (SymbolsDataTable symbolTable = new SymbolsDataTable())
            using (SymbolsTableAdapter symbolAdapter = new SymbolsTableAdapter())
            {
                symbolAdapter.FillByActiveSymbols(symbolTable, market);
                var query = from c in symbolTable
                            select c.Symbol;
                return query.AsParallel().ToList();
            }
    }
public static void getHistoricalStockData(string[] symbols, string market)
    {
        // download data for list of symbols and then upload to db tables
        Uri uri;
        string url;
        List<StockData> results = new List<StockData>();
        System.Net.ServicePointManager.Expect100Continue = false;
        System.Net.ServicePointManager.DefaultConnectionLimit = 1000;
        DailyAmexDataDataTable tempTable = new DailyAmexDataDataTable();
        Parallel.ForEach(symbols, async symbol =>
            {
                    url = "http://ichart.finance.yahoo.com/table.csv?s=" + symbol + "&a=00&b=1&c=1900&d=" + (DateTime.Now.Month - 1) + "&e=" + DateTime.Now.Day + "&f=" + DateTime.Now.Year + "&g=d&ignore=.csv";
                    uri = new Uri(url);
                    HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(uri);
                    webRequest.KeepAlive = true;
                    webRequest.ProtocolVersion = HttpVersion.Version10;
                    webRequest.ServicePoint.ConnectionLimit = 24;
                    webRequest.Timeout = 10000;
                    webRequest.Headers.Add("UserAgent", "Pentia; MSI");
                    Stopwatch watch = new Stopwatch();
                    watch.Start();
                    WebResponse webResponse = await webRequest.GetResponseAsync();
                    watch.Stop();
                    Debug.WriteLine(watch.ElapsedMilliseconds);
                    using (DailyAmexDataDataTable table = new DailyAmexDataDataTable())
                    using (DailyAmexDataTableAdapter adapter = new DailyAmexDataTableAdapter())
                    using (Stream stream = webResponse.GetResponseStream())
                    using (StreamReader reader = new StreamReader(stream))
                    {
                        adapter.Adapter.UpdateBatchSize = 1000;
                                adapter.ClearBeforeFill = true;
                        try
                        {
                             adapter.FillBySymbol(table, symbol);
                        }
                        bool breakLoop = false;
                        while (breakLoop == false && reader.EndOfStream == false)
                        {
                            Object lockDate = new Object();
                            lock (lockDate)
                            {
                                string[] lineArray;
                                string line;
                                line = reader.ReadLine();
                                lineArray = line.Split(',');
                                DateTime testDate = new DateTime();
                                bool isDate = DateTime.TryParse(lineArray[0], out testDate);
                                decimal open = 0, high = 0, low = 0, close = 0, adjClose = 0;
                                DateTime date = new DateTime();
                                Int64 volume = 0;
                                // if it isn't the very first line
                                if (isDate)
                                {
                                    // set the data for each array here
                                    DateTime.TryParse(lineArray[0], out date);
                                    Decimal.TryParse(lineArray[1], out open);
                                    Decimal.TryParse(lineArray[2], out high);
                                    Decimal.TryParse(lineArray[3], out low);
                                    Decimal.TryParse(lineArray[4], out close);
                                    Int64.TryParse(lineArray[5], out volume);
                                    Decimal.TryParse(lineArray[6], out adjClose);
                                    var amexQuery = from c in table
                                                            where c.Date == date
                                                            select new StockData { Close = c.AdjustedClose };
                                    results = amexQuery.AsParallel().ToList();
                                    if (results != null)
                                    {
                                        breakLoop = true;
                                    }
                                    else
                                    {
                                        adapter.Insert(symbol, open, high, low, close, volume, adjClose, date);
                                    }
                                }
                            }
                        }
                        tempTable = (DailyAmexDataDataTable)table.GetChanges();
                        if (tempTable != null)
                        {
                            adapter.Update(tempTable);
                            tempTable.Dispose();
                        }
                    }
        );
    }