Slower WPF Performance after a button gets focus

using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;

namespace ButtonSpeedTest
{
 public partial class Window1 : Window
 {
  bool isDragging;

  public Window1()
  {
   InitializeComponent();
  }

  private void MyMouseDown(object sender, MouseButtonEventArgs e)
  {
   isDragging = true;
  }

  private void MyMouseUp(object sender, MouseButtonEventArgs e)
  {
   isDragging = false;
  }  

  private void MyMouseMove(object sender, MouseEventArgs e)
  {
   if (isDragging)
   {
    Point position = e.GetPosition(this);

    rect.SetValue(Canvas.LeftProperty, position.X - rect.Width / 2);
    rect.SetValue(Canvas.TopProperty, position.Y - rect.Height / 4);
   }
  }
 }
}