Writing GetResponseStream to file is taking long

I have made a SSRS Bulk reporter that queries a report from SSRS using a HTTPWebRequest and a HTTPWebResponse to get the stream and save it as a file to a PDF.

However, the client has complained that this is taking long. I have tested it, and this function is taking +- 15.5seconds. If I put a break in-between the GetResponseStream and the file-writer, and I wait for about 3 seconds before stepping to the next part, it shaves 4 seconds off the total time?

Can anyone explain this/or give some advise as to make it a bit faster?

This is the function:

 Public Function ExportReport(ByVal QueryStringParameters As String, ByVal FileName As String) As String
    Dim PDFName As String = ReportFolderPath & FileName & ".pdf"
    'Create a http request or connecting to the report server.
    Dim ReportHTTPRequest As HttpWebRequest = Nothing
    'Create a http response to catch the data stream returned from the http request.
    Dim ReportHTTPResponse As HttpWebResponse = Nothing
    'Create a stream to read the binary data from the http reponse.
    Dim ReportStream As Stream = Nothing
    Dim ReportFileStream As FileStream = Nothing
    'Create an array of bytes to get the binary data from the stream.
    Dim ReportBytes As Byte()
    Dim ReportBuffer As Integer = 204800
    Dim ReportBytesRead As Integer = 0


    'Create a webrequest to get the report with all the report parameters included.
    ReportHTTPRequest = WebRequest.Create(ReportServerURL & QueryStringParameters)
    ReportHTTPRequest.Credentials = CredentialCache.DefaultCredentials
    'Get the response from the request.

    ReportHTTPResponse = ReportHTTPRequest.GetResponse()

    'Read the binary stream from the http response.
    ReportStream = ReportHTTPResponse.GetResponseStream()

    ReportBytes = New Byte(ReportBuffer) {}
    ReportBytesRead = ReportStream.Read(ReportBytes, 0, ReportBuffer)
    ReportFileStream = New FileStream(PDFName, FileMode.Create)
    Do While ReportStream.CanRead And ReportBytesRead > 0
        ReportFileStream.Write(ReportBytes, 0, ReportBytesRead)
        ReportBytesRead = ReportStream.Read(ReportBytes, 0, ReportBuffer)
    Loop

    ReportHTTPResponse.Close()
    ReportStream.Close()
    ReportFileStream.Close()

    Return PDFName
End Function

a magic thing that worked for me, try a buffer size of 32768