Capture Screenshots in a Row

I would like to capture a continuous moving action in Unity, so that I could use those screenshots to post-produce a movie through library like FFMpeg on Android. By referencing this article I do captured screenshots I want. However, when the action goes more complex and the frames increases, the whole thread seems freeze and has no any response.

I got so many log output from unity like:

 D/Unity   (29568): Watchdog wakeup : 3.98s has past without activity
 D/Unity   (29568): Watchdog wakeup : 5.36s has past without activity
 D/Unity   (29568): Watchdog wakeup : 6.63s has past without activity
 D/Unity   (29568): Watchdog wakeup : 8.52s has past without activity
 D/Unity   (29568): Watchdog wakeup : 9.96s has past without activity
 D/Unity   (29568): Watchdog wakeup : 5.91s has past without activity

My code snippet which take the screenshot and saved it :

 public IEnumerator CaptureScreen (float duration)
     {
         // calculate how much frames taken by this frame under currently set fps
         int targetFrames = (int) Math.Round (duration * captureFPS);
         
         renderTexture = new RenderTexture(Screen.width, Screen.height, 24);
         captureCam.targetTexture = renderTexture;
         
         Texture2D texture2d = new Texture2D(cropWidth, cropHeight, TextureFormat.RGB24, false);
         
         captureCam.Render();
         RenderTexture.active = renderTexture;
         
         texture2d.ReadPixels(new Rect(0, 0, cropWidth, cropHeight), 0, 0);
         
         RenderTexture.active = null;
         
         byte[] bytes;
         yield return bytes = texture2d.EncodeToPNG();
         
         Destroy(texture2d);
         // get target numbers from the same frame.
         for (int i = 0; i < targetFrames; i++)
         {
             ++frameCounter;
             // full file path, generated by Application.persistentDataPath + folder & filename
             string filePath = Utils.AnimeFrameName(frameCounter);
             
             using( FileStream fs = new FileStream(filePath, FileMode.Create, FileAccess.Write) )
             {
                 fs.Write(bytes, 0, bytes.Length);
                 yield return fs;
                 Debug.Log("Screenshot to =>>> " + filePath);
             }
         }
     }

Have no idea why the whole progress just freeze but not crash and force-close the app.