Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    If Downloading Then Exit Sub
    Downloading = True

    Dim wc As New WebClient
    AddHandler wc.DownloadProgressChanged, AddressOf wc_ProgressChanged
    AddHandler wc.DownloadFileCompleted, AddressOf wc_DownloadDone

    wc.DownloadFileAsync(New Uri("http://speedtest.ftp.otenet.gr/files/test100Mb.db"), tmp, Stopwatch.StartNew)
End Sub

Private Sub wc_DownloadDone(sender As Object, e As System.ComponentModel.AsyncCompletedEventArgs)
    Downloading = False
End Sub

Private Sub wc_ProgressChanged(sender As Object, e As DownloadProgressChangedEventArgs)
    Me.Label2.Text = (e.BytesReceived / (DirectCast(e.UserState, Stopwatch).ElapsedMilliseconds / 1000.0#)).ToString("#")
End Sub