Error in gzip http request and response

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HTTP

My Simple Xml Http Request and response taking 150 to 200 Second for resolve this I try to gzip http Request and response, with below code then getting Error. The remote server returned an error: (500) Internal Server Error.

Please Help me, how to solve this problem

Protected Function PostXml(ByVal url As String, ByVal xml As String) As String
        Dim strResult As String = String.Empty
        Dim request As HttpWebRequest = DirectCast(WebRequest.Create(url), HttpWebRequest)

        request.Headers.Add(HttpRequestHeader.AcceptEncoding, "gzip,deflate")
        
If Not String.IsNullOrEmpty(xml) Then
            request.Method = "POST"
            Dim lbPostBuffer As Byte() = Encoding.[Default].GetBytes(xml)
            request.ContentLength = lbPostBuffer.Length
            Dim PostStream As Stream = request.GetRequestStream()
            PostStream.Write(lbPostBuffer, 0, lbPostBuffer.Length)
            PostStream.Close()
        End If
       
 Try
            Using WebResponse As HttpWebResponse = DirectCast(request.GetResponse(), HttpWebResponse)
                'Dim WebResponse As HttpWebResponse = DirectCast(request.GetResponse(), HttpWebResponse)
                Dim responseStream As Stream = WebResponse.GetResponseStream()
               
 If WebResponse.ContentEncoding.ToLower().Contains("gzip") Then
                    responseStream = New GZipStream(responseStream, CompressionMode.Decompress)
                ElseIf WebResponse.ContentEncoding.ToLower().Contains("deflate") Then
                    responseStream = New DeflateStream(responseStream, CompressionMode.Decompress)
                End If
               
 Dim Reader As New StreamReader(responseStream, Encoding.[Default])
                Dim Html As String = Reader.ReadToEnd()
                WebResponse.Close()
                responseStream.Close()
                strResult = Html
            End Using
        Catch ex As WebException
            strResult = ""
        End Try
        Return strResult
    End Function