！PictureBox 用Emgu CV实现播放视频，播放速度明显加快，如何恢复成原视频播放速度？

求助！PictureBox 用Emgu CV实现播放视频，播放速度明显加快，怎么恢复成原视频播放速度？？？

        /// <summary>
        /// 打开视频文件
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void btnVideo_Click(object sender, EventArgs e)
        {
            OpenFileDialog opd = new OpenFileDialog();//对话框
            opd.Filter = "AVI(*.avi)|*.avi|MP4(*.mp4)|*.mp4|WMV(*.wmv)|*.wmv|3GP(*.3gp)|*.3gp|MKV(*.mkv)|*.mkv|RM(*.rm)|*.rm|RMVB(*.rmvb)|*.rmvb|FLV(*.flv)|*.flv|VOB(*.vob)|*.vob|MPEG(*.mpeg)|*.mpeg|MOV(*.mov)|*.mov";//设置过滤条件
            if (opd.ShowDialog() == DialogResult.OK)//用户是否单击了“打开”按钮
            {
                string file = opd.FileName;
                c = new Capture(file);
                aa();
            }
        }

  public delegate void PlayVideoDelegate();

        public delegate void RefreshPictureBoxDelegate(Bitmap frame);

       Emgu.CV.Capture c;

        public void aa()
        {
            PlayVideoDelegate play = this.PlayVideoFile;
            IAsyncResult status = play.BeginInvoke(null, null);
        }

        /// <summary>  
        /// 播放视频文件
        /// </summary>  
        private void PlayVideoFile()
        {
            while (true)
            {
                Image<Bgr, byte> frame = c.QueryFrame();
                if (frame != null)
                {
                    Image<Gray, byte> grayFrame = frame.Convert<Gray, byte>();
                    grayFrame.Resize(this.Width, this.Height, INTER.CV_INTER_CUBIC);
                    RefreshPictureBoxDelegate fresh = this.RefreshPictureBox;
                    try
                    {
                        this.BeginInvoke(fresh, grayFrame.ToBitmap());
                    }
                    catch (ObjectDisposedException ex)
                    {
                        Thread.CurrentThread.Abort();
                    }
                }
                else
                {
                    break;
                }
            }
        }

        /// <summary>  
        /// 刷新UI线程的pixtureBox的方法。  
        /// </summary>  
        /// <param name="frame">要刷新的帧。</param>  
        private void RefreshPictureBox(Bitmap frame)
        {
            this.picBoxView.Image = frame;
        }  

------解决思路----------------------
while (true)
            {
                Image<Bgr, byte> frame = c.QueryFrame();
                if (frame != null)
                {
                    Image<Gray, byte> grayFrame = frame.Convert<Gray, byte>();
                    grayFrame.Resize(this.Width, this.Height, INTER.CV_INTER_CUBIC);
                    RefreshPictureBoxDelegate fresh = this.RefreshPictureBox;
                    try
                    {
                        this.BeginInvoke(fresh, grayFrame.ToBitmap());
                    }
                    catch (ObjectDisposedException ex)
                    {
                        Thread.CurrentThread.Abort();
                    }
                }
                else
                {
                    break;
                }
               Thread.Sleep(50);//每秒20帧
            }