WPF app struggling with dispatcher timer

Question

I switched from windows forms to WPF recently and I made a simple four second timer. However, the timer looks like it is struggling, it has 100 milisecond interval, but when it tries to display value it just struggles, like display, stop for 0.2 seconds, display, stop etc. Here is my code:

private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            prg.Height = 0;
            System.Windows.Threading.DispatcherTimer dispatcherTimer = new System.Windows.Threading.DispatcherTimer();
            dispatcherTimer.Tick += dispatcherTimer_Tick;
            dispatcherTimer.Interval = new TimeSpan(0, 0, 0, 0, 100);
            dispatcherTimer.Start();

            this.MouseDown += delegate { DragMove(); };
        }

private void dispatcherTimer_Tick(object sender, EventArgs e)
        {
            TimeSpan timeElapsed = DateTime.Now - startTime;
            time = timeElapsed.TotalSeconds.ToString("0.0");//Time elapsed to string
            label1.Content = time; //Display time elapsed on label
            secs = (float)Convert.ToDouble(time); //Convert from string to float for measure
            prg.Height = (secs / 4.0) * 117; //Height of "progress" bar
            if(secs >= 4.0) //If time elapsed more than 4 seconds reset the timer
            {
                startTime = DateTime.Now;
                // Imaging lower dont bother
                seq++;
                if (seq == 1)
                {
                    coei.Source = ImageSourceFromBitmap(Properties.Resources.convvice2);
                }
                if (seq == 2)
                {
                    coei.Source = ImageSourceFromBitmap(Properties.Resources.convvfire2);
                }
                if (seq == 3)
                {
                    coei.Source = ImageSourceFromBitmap(Properties.Resources.convvlight);
                }
                if (seq == 4)
                {
                    coei.Source = ImageSourceFromBitmap(Properties.Resources.convvphyd);
                }
                if (seq == 5)
                {
                    seq = 1;
                    coei.Source = ImageSourceFromBitmap(Properties.Resources.convvice2);
                }
            }
        }

Answer 1

I made some additions to the code you provided and did not reproduce your problem.
Xaml part :

 <Grid>  
        <ProgressBar Name="prg" HorizontalAlignment="Left" Height="10" Margin="138,58,0,0" VerticalAlignment="Top" Width="100"/>  
        <Label Name="label1" Content="" HorizontalAlignment="Left" Margin="327,58,0,0" VerticalAlignment="Top" RenderTransformOrigin="5.441,-2.026"/>  
        <StackPanel Background="Azure" Height="100" Width="100">  
            <Image Name="coei" HorizontalAlignment="Left" Height="100" VerticalAlignment="Top" Width="100" Source="/paste.bmp"/>  
  
        </StackPanel>  
        <Label Name="lb2" Content="0" HorizontalAlignment="Left" Margin="502,59,0,0" VerticalAlignment="Top"/>  
  
    </Grid>  

C# code:

  //....Your code Window_Loaded  
        DateTime startTime = DateTime.Now;  
        string time;  
        double secs;  
        int seq=0;  
        private void dispatcherTimer_Tick(object sender, EventArgs e)  
        {  
                //....Your code  
                coei.Stretch = Stretch.Fill;  
                lb2.Content = seq;  
            }  
        }  
  
        public BitmapImage ImageSourceFromBitmap(string uri)  
        {  
            BitmapImage bi3 = new BitmapImage();  
            bi3.BeginInit();  
            bi3.UriSource = new Uri(uri, UriKind.RelativeOrAbsolute);  
            bi3.EndInit();  
            return bi3;  
        }  

The result picture is:

Can you tell me which step I missed to reproduce your question?

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