You could do it like this.
This example downloads a web page, measures the time needed
and then calculates the speed in bytes per seconds.
Stopwatch stopwatch = new Stopwatch();
stopwatch.Reset();
stopwatch.Start();
WebClient webClient = new WebClient();
byte[] bytes = webClient.DownloadData("http://www.codeproject.com");
stopwatch.Stop();
double seconds = stopwatch.Elapsed.TotalSeconds;
double speed = bytes.Count() / seconds;
MessageBox.Show(string.Format("Your speed: {0} bytes per second.", speed));