CancellationToken callback was not executed immeditely

WPF application,  I have

 public  CancellationTokenSource cTokenSource;
 public  static CancellationToken cToken;
 cTokenSource = new CancellationTokenSource();
 cToken = cTokenSource.Token;

 async Task Consumer()
 {
     try
     {
        dddd d = new dddd(ts);
        Dictionary<string, string> dict =  d.RunScript(cToken);

Then in a cancel button event.

private void Cancel_Click(object sender, RoutedEventArgs e)
 {
      cTokenSource.Cancel();
 }

I registered the call back in RunScript method.

 string s = "a very long paragraph";
m_VoiceResource.PlayTTS(s);
cToken.Register(() =>
      {
         m_VoiceResource.Stop(); // stop any activity on m_VoiceResource
      }

However after I clicking the cancel button, the code didn't reach m_VoiceResource.Stop() immediately. I want to interrupt the play text to speech, which is PlayTTS.  Did I block any thread? I don't think so but why?