Amazon S3 Multipart UploadPartRequest allows only single thread to upload at the same time using asp.net

I am trying to upload video files amazon s3 using Multipart upload method in asp.net and I traced the upload progress using logs. It uploads 106496 each time and runs only single thread at a time.I did not notice that multiple threads running.Please clarify me on this why it is running single thread and it's taking long time to upload even for 20Mb file it's taking almost 2 minutes.

Here is my code,which uses UploadPartRequest.

private void UploadFileOnAmazon(string subUrl, string filename, Stream audioStream, string extension) { client = new AmazonS3Client(accessKey, secretKey, Amazon.RegionEndpoint.USEast1);

        // List to store upload part responses.
        List<UploadPartResponse> uploadResponses = new List<UploadPartResponse>();


        // 1. Initialize.
        InitiateMultipartUploadRequest initiateRequest = new InitiateMultipartUploadRequest
        {
            BucketName = bucketName,
            Key = subUrl + filename
        };

        InitiateMultipartUploadResponse initResponse =
            client.InitiateMultipartUpload(initiateRequest);

        // 2. Upload Parts.
        //long contentLength = new FileInfo(filePath).Length;
        long contentLength = audioStream.Length;
        long partSize = 5 * (long)Math.Pow(2, 20); // 5 MB


        try
        {
            long filePosition = 0;
            for (int i = 1; filePosition < contentLength; i++)
            {

                UploadPartRequest uploadRequest = new UploadPartRequest
                {
                    BucketName = bucketName,
                    Key = subUrl + filename,
                    UploadId = initResponse.UploadId,
                    PartNumber = i,
                    PartSize = partSize,
                    FilePosition = filePosition,
                    InputStream = audioStream
                    //FilePath = filePath
                };

                // Upload part and add response to our list.
                uploadRequest.StreamTransferProgress += new EventHandler<StreamTransferProgressArgs>(UploadPartProgressEventCallback);                 
                uploadResponses.Add(client.UploadPart(uploadRequest));

                filePosition += partSize;
            }

            logger.Info("Done");


            // Step 3: complete.
            CompleteMultipartUploadRequest completeRequest = new CompleteMultipartUploadRequest
            {
                BucketName = bucketName,
                Key = subUrl + filename,
                UploadId = initResponse.UploadId,
                //PartETags = new List<PartETag>(uploadResponses)

            };
            completeRequest.AddPartETags(uploadResponses);
            CompleteMultipartUploadResponse completeUploadResponse =
                client.CompleteMultipartUpload(completeRequest);

        }
        catch (Exception exception)
        {
            Console.WriteLine("Exception occurred: {0}", exception.Message);
            AbortMultipartUploadRequest abortMPURequest = new AbortMultipartUploadRequest
            {
                BucketName = bucketName,
                Key = subUrl + filename,
                UploadId = initResponse.UploadId
            };
            client.AbortMultipartUpload(abortMPURequest);
        }
    }
    public static void UploadPartProgressEventCallback(object sender, StreamTransferProgressArgs e)
    {
        // Process event. 
        logger.DebugFormat("{0}/{1}", e.TransferredBytes, e.TotalBytes);
    }       

Is there anything wrong with my code or how to make threads run simultaneously to speed up upload?