Is my Excel data extraction not running efficiently?

I'm writing a console app that will extract data from many xls files and lump them into one large csv. I have all the Excel application loading/GC done between each file and I worry it's causing the application to take longer than needed.

Here is my code:

namespace SubconDataExtractToCSV
{
    class Program
    {
        static void Main(string[] args)
        {
            StreamWriter outputCSV = new StreamWriter(@"C:\Users\mbelmer\Desktop\Helpful Files\Subcon Files\SubconEmailExtract\ExtractData\ExtractedData.csv", true);
            DirectoryInfo folder = new DirectoryInfo(@"C:\Users\mbelmer\Desktop\Helpful Files\Subcon Files\SubconEmailExtract\");
            FileInfo[] files = folder.GetFiles("*.xls");
            foreach (var file in files)
            {
                ExtractData(file, outputCSV);
            }
            outputCSV.Close();
        }

        static void ExtractData(FileInfo filename, StreamWriter output)
        {
            Excel.Application xlApp = new Excel.Application();
            Excel.Workbook xlWorkbook = xlApp.Workbooks.Open(filename.FullName);
            Excel._Worksheet xlWorksheet = xlWorkbook.Sheets[1];
            Excel.Range xlRange = xlWorksheet.UsedRange;
            int rowCount = xlRange.Rows.Count;
            int colCount = xlRange.Columns.Count;
            string fileDate = filename.Name.Substring(0, 7);
            if (filename.Name.Contains("WIP"))
            {
                //EXTRACT MMT
                for(int i = 2; i <= rowCount; i++)
                {
                    string dataLine = fileDate + ",";
                    for(int j = 1; j <= 10; j++)
                    {
                        if (xlRange.Cells[i, j].Value2 != null)
                        {
                            dataLine += xlRange.Cells[i, j].Value2 + ",";
                        }
                        else
                        {
                            dataLine += ",";
                        }
                    }
                    output.WriteLine(dataLine);
                }

            }
            else
            {
                //EXTRACT AMKOR
                for(int i = 2; i <= rowCount; i++)
                {
                    string dataLine = fileDate + ",,,,,,,,,,,";
                    for(int j = 1; j <= colCount; j++)
                    {
                        if(xlRange.Cells[i,j].Value2 != null)
                        {
                            dataLine += xlRange.Cells[i, j].Value2 + ",";
                        }
                        else
                        {
                            dataLine += ",";
                        }
                    }
                    output.WriteLine(dataLine);
                }
            }
            GC.Collect();
            GC.WaitForPendingFinalizers();
            Marshal.ReleaseComObject(xlRange);
            Marshal.ReleaseComObject(xlWorksheet);
            xlWorkbook.Close();
            Marshal.ReleaseComObject(xlWorkbook);
            xlApp.Quit();
            Marshal.ReleaseComObject(xlApp);
        }
    }
}

Should I put the application open and close outside the initial foreach? It's currently going through 38 files and takes just over 6 minutes to do so. Thank you!