FileStream Object taking forever to push file to browser - asp.net

Question

I'm using a filestream object to push a report in excel format from our DB, and it works fine for reports that are reasonable in terms the number of rows, however there's one report that has over 15K rows and the site times out after 20 minutes - I know I could increase the timeout, but even 20 minutes is unaccaptable right now. Is there any way to speed things up? Here is my current code:
string path = Server.MapPath(#"~\");
string sourceName = Path.Combine(path, "Demo", "Report_Template", templateName);
string newFileName = Path.Combine(Global.demo_data_directory, #"Reports", #"Demo Download Activity Report.xlsx"); // Path.Combine(path, "Demo_Data", "Reports", Filename + ".xlsx");
string copyresults = Utilities.CopyFile(sourceName, newFileName);
if (copyresults != "Copied file")
{
DemoDAL.ErrorLog("GenerateDownloadActivityReportFile.Page_Load: Copy Error - " + copyresults);
}
else
{
document = SpreadsheetDocument.Open(newFileName, true);
wbPart = document.WorkbookPart;
DataTable datatbl = (DataTable)Session["gvPOC"];
CreateReport(datatbl);
// ADDED THIS *********************************************************************************************************
//wbPart.Workbook.Save();
using (FileStream fsSource = new FileStream(newFileName, FileMode.Open, FileAccess.Read, FileShare.Read, 10240))
{
// Read the source file into a byte array.
byte[] bytes = new byte[fsSource.Length];
int numBytesToRead = (int)fsSource.Length;
int numBytesRead = 0;
while (numBytesToRead > 0)
{
// Read may return anything from 0 to numBytesToRead.
int n = fsSource.Read(bytes, numBytesRead, numBytesToRead);
// Break when the end of the file is reached.
if (n == 0)
break;
numBytesRead += n;
numBytesToRead -= n;
}
numBytesToRead = bytes.Length;
StreamFileToBrowser(newFileName, bytes);
}
File.Delete(newFileName);
}
public void StreamFileToBrowser(string sfilename, byte[] fileBytes)
{
try
{
Response.Clear();
Response.ClearHeaders();
Response.AppendHeader("Content-disposition", String.Format("attachment; filename=\"{0}\"", System.IO.Path.GetFileName(sfilename)));
Response.AppendHeader("Content-Type", "binary/octet-stream");
Response.AppendHeader("Content-length", fileBytes.Length.ToString());
Response.BinaryWrite(fileBytes);
if (Response.IsClientConnected)
Response.Flush();
}
catch (Exception ex)
{
DemoDAL.ErrorLog("StreamFileToBrowser: " + ex.Message);
}
}