Why C# List<T> Add method is very slow : I have asp.net mvc project with dapper read data from database and I need export to excel.

Dapper is fast ! `ExecuteReader` only 35 seconds.

But `list.Add(InStock);` spend too much time !!! Over 1020 seconds !!

Is there any idea ?? Thank you !!

        public List<InStock> GetList(string stSeId, string edSeId, string stSeDay, string edSeDay, string qDate)
        {
            List<InStock> list = new List<InStock>();
            InStock InStock = null;

            IDataReader reader;

            using (var conn = _connection.GetConnection())
            {
                try
                {
                    conn.Open();
                    //******************Only 35 seconds*****
                    reader = conn.ExecuteReader(fileHelper.GetScriptFromFile("GetInStock"),
                         new { STSeId = stSeId, EDSeId = edSeId, STSeDay = stSeDay, EDSeDay = edSeDay, qDate = qDate });
                    //*************************************

                    //******************Over 1020 seconds**********
                    while (reader.Read())
                    {
                        InStock = new InStock();

                        InStock.ColA = reader.GetString(reader.GetOrdinal("ColA"));
                        InStock.ColB = reader.GetString(reader.GetOrdinal("ColB"));
                        InStock.ColC = reader.GetString(reader.GetOrdinal("ColC"));

                        list.Add(InStock);
                    }
                    //*********************************************
                    return list;
                }
                catch (Exception err)
                {
                    throw err;
                }

            }
        }


  [1]: https://i.stack.imgur.com/gJJ1k.png