How to quickly parse links on the portal?

Question

Parsing

You need to find broken links on the portal.
I download the html page, find all the links on it and add them to the download queue. In the process, I highlight broken / incorrect.

HttpWebResponse response;
                StreamReader respStream;
                try
                {
                    HttpWebRequest request = (HttpWebRequest)WebRequest.Create(link.ToString());
                    request.UserAgent = "Mozilla/5.0 (Windows NT 6.1; rv:11.0) Gecko/20100101 Firefox/15.0";
                    request.Accept = "Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8";
                    request.AllowAutoRedirect = true;
                    request.Timeout = 10000;
                    response = (HttpWebResponse)request.GetResponse();
                    respStream = new StreamReader(response.GetResponseStream());
                    html = respStream.ReadToEnd();
                    response.Close();
                    respStream.Close();
                }
                catch (Exception ex)
                {
                    System.Console.WriteLine("-------------\n" +
                            "Bad link: " + link + "\n" +
                            "From: " + link.Parent +
                            "\n" + ex.Message);
                    link.ErrorComments = ex.Message;
                    link.Parent.AddSon(link);
                    continue;
                }

Here is the download code.
I parallelized the program, but the speed has not changed globally. From 60 links per minute in a single thread, the speed has increased to 120. Maybe I'm not optimally downloading pages? Internet speed allows you to download 10-20 pages per second. At the same time, the download speed decreases over time. How to speed up the process?