webclient异步下载的问题


protected void getTaskList(string keyword, string number)
        {
            List<Data> list = new List<Data>();
            List<ArrayList> arrayListList = new List<ArrayList>();
            ArrayList arrayList = new ArrayList();
            string path = Application.StartupPath + "\\temp\\" + keyword + "\\";
            keyword = HttpUtility.UrlEncode(keyword, Encoding.GetEncoding(936)).ToUpper();
            int pn = 0;
            while (pn < Int32.Parse(number))
            {
                string url = @"xxxxxxxxxx";
                pn += 20;
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
                HttpWebResponse response = (HttpWebResponse)request.GetResponse();
                if (response.StatusCode == HttpStatusCode.OK)
                {
                    using (Stream ReceiveStream = response.GetResponseStream())
                    {
                        using (StreamReader Reader = new StreamReader(ReceiveStream))
                        {
                            string json = Reader.ReadToEnd();
                            JsonResult Result = JsonConvert.DeserializeObject<JsonResult>(json);
                            if (Result != null)
                            {
                                if (Result.data.Count > 0)
                                {
                                    list.AddRange(Result.data);
                                }
                            }
                        }
                    }
                }
            }
            if (list.Count > 0)
            {
                if (!File.Exists(path)) Directory.CreateDirectory(path);
                for (int i = 0; i < list.Count; i++)
                {
                    if (string.IsNullOrEmpty(list[i].objURL)) continue;
                    arrayList = new ArrayList() { i, list[i].fromPageTitleEnc, list[i].filesize, "0 KB/S", "0%", list[i].objURL, path + Guid.NewGuid().ToString() + "." + list[i].type, true };
                    arrayListList.Add(arrayList);
                }
                this.dgvDownLoad.Invoke(new Action<int, object>(this.CallBackMethod), 1, arrayListList);
            }
        }这段是我从一个地址获取数据的代码，然后批量下载文件（都是图片文件单个10k-30k不等），一开始下载速度挺快的，每次下载10来个就速度奇慢无比。void StartDownLoad(object o)
        {
            SynFileInfo m_SynFileInfo = (SynFileInfo)o;
            m_SynFileInfo.LastTime = DateTime.Now;
            //再次new 避免WebClient不能I/O并发 
            WebClient client = new WebClient();
            if (m_SynFileInfo.Async)
            {
                //异步下载
                client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(client_DownloadProgressChanged);
                client.DownloadFileCompleted += new AsyncCompletedEventHandler(client_DownloadFileCompleted);
                client.DownloadFileAsync(new Uri(m_SynFileInfo.DownPath), m_SynFileInfo.SavePath, m_SynFileInfo);
            }
            else client.DownloadFile(new Uri(m_SynFileInfo.DownPath), m_SynFileInfo.SavePath);
        }这里是异步方法，请问是什么原因呢？