为什么在 while 循环中和在 while 循环中构造字典之间存在显著的性能差异？

问：

当我从大型 csv 文件中读取数据时，我面临着奇怪的性能差异。如果我在同一循环中读取数据并构造字典，如下面的代码片段所示，则该方法将花费大约 4.1 秒来完成该过程。

private void ReadFileWorkerRun(object sender, EventArgs e)
{
    List<Stock> lineTemp = new List<Stock>();
    List<Stock> allStock = new List<Stock>();
    List<List<Stock>> orderedAll = new List<List<Stock>>();
    Categories = new Dictionary<string, List<Stock>>() { { GlobalVariable.ALL, allStock } };
    DictionaryOrder = new List<(string, string)>();

    using (StreamReader lines = new StreamReader(FilePath))
    {
        string line = lines.ReadLine();

        // Add each stock to dictionary
        while ((line = lines.ReadLine()) != null)
        {
            Stock temp = new Stock(line);

            // This is the upper boundary of the code that will move outside of the using statement
            if (!Categories.TryGetValue(temp.StockID, out List<Stock> targetList))
            {
                targetList = new List<Stock>();
                orderedAll.Add(targetList);
                Categories.Add(temp.StockID, targetList);
                DictionaryOrder.Add((temp.StockID, temp.StockName));
            }
            targetList.Add(temp);
            // This is the lower boundary of the code that will move outside of the using statement
        }
    }
    /*
    The code between the boundry is moved here
    */
    foreach (List<Stock> stockList in orderedAll)
    {
        allStock.AddRange(stockList);
    }
}

public class Stock
    {
        public string StockDate { get; set; }

        public string StockID { get; set; }

        public string StockName { get; set; }

        public string SecBrokerID { get; set; }

        public string SecBrokerName { get; set; }

        public decimal Price { get; set; }

        public long BuyQty{ get; set; }

        public long SellQty { get; set; }

        public Stock(string s)
        {
            string[] data = s.Split(',');
            StockDate = data[0];
            StockID = data[1];
            StockName = data[2];
            SecBrokerID = data[3];
            SecBrokerName = data[4];
            Price = decimal.Parse(data[5]);
            BuyQty = long.Parse(data[6]);
            SellQty = long.Parse(data[7]);
        }
    }

但是，当我将构造字典的代码部分移出 while 循环并将其放入 foreach 循环中时，该方法所花费的时间将变为 3.4 秒。using 语句中的代码分为以下代码：

using (StreamReader lines = new StreamReader(FilePath))
{
    string line = lines.ReadLine();

    while ((line = lines.ReadLine()) != null)
    {
            lineTemp.Add(new Stock(line));
    }
}

// Add each stock to dictionary
foreach (Stock temp in lineTemp)
{
    if (!Categories.TryGetValue(temp.StockID, out List<Stock> targetList))
    {
        targetList = new List<Stock>();
        orderedAll.Add(targetList);
        Categories.Add(temp.StockID, targetList);
        DictionaryOrder.Add((temp.StockID, temp.StockName));
    }
    targetList.Add(temp);
}

两个版本之间的唯一区别是我在第二部分中列出的代码，无论我运行多少次，时间间隔始终是一致的，那么为什么具有相同逻辑和数据结构的代码的行为如此不同呢？