在C＃中有效地将.xslx转换为.csv？

Question

作为输入，我有一组excel文件，里面有几个工作表。我需要为每个工作表导出一个csv文件。下面是我的代码，但它很慢。它建立在这个previous post中提出的解决方案的基础之上。请考虑我必须在相当大的.xlsx文件（大约300Mb）上运行它。

问题：有什么办法可以改善这个吗？

void Main()
{
        string folder = @"\\PATH_TO_FOLDER\";

        var files = Directory.GetFiles(folder, "*.xlsx", SearchOption.TopDirectoryOnly);

        foreach (string file in files)
        {

            ConvertToCsv(file,  Directory.GetParent(file) + @"\\output\");
        }


}

public static void ConvertToCsv(string file, string targetFolder)
{

    FileInfo finfo = new FileInfo(file);
    ExcelPackage package = new ExcelPackage(finfo);

    // if targetFolder doesn't exist, create it
    if (!Directory.Exists(targetFolder)) {
        Directory.CreateDirectory(targetFolder);
    }


    var worksheets = package.Workbook.Worksheets;
    int sheetcount = 0;

    foreach (ExcelWorksheet worksheet in worksheets)
    {
        sheetcount++;
        var maxColumnNumber = worksheet.Dimension.End.Column;
        var currentRow = new List<string>(maxColumnNumber);
        var totalRowCount = worksheet.Dimension.End.Row+1;
        var currentRowNum = 1;


        //No need for a memory buffer, writing directly to a file
        //var memory = new MemoryStream();

        string file_name = targetFolder +  Path.GetFileNameWithoutExtension(file) +  "_" + sheetcount + ".csv";

        using (var writer = new StreamWriter(file_name, false, Encoding.UTF8))
        {
            //the rest of the code remains the same


            for (int i = 1; i < totalRowCount; i++)
            {
                i.Dump();
                // populate line with semi columns separators
                string line = "";
                for (int j = 1; j < worksheet.Dimension.End.Column+1; j++)
                {
                    if (worksheet.Cells[i, j].Value != null)
                    {
                        string cell = worksheet.Cells[i, j].Value.ToString() + ";";
                        line += cell;
                    }
                }
                // write line
                writer.WriteLine(line);
            }
        }
    }
}