WPF UI interaction lagging, but why?

Question

To demonstrate a problem I'm having, I've coded the simple example below that draws a curve, and a black line that follows the cursor.

Can someone explain why when I increase "numPointsMul", interaction with the mouse and the drawing of the cursor line becomes very laggy?

Why is the number of points in the graph affecting the drawing of the pointer in this way? Notice I have turned off all hit testing, and the graph is only being redrawn in the OnRender.

I gather that increasing the number of points would cause the OnRender function to take longer, but I don't see why it would cause the drawing of the pointer to lag, as OnRender isn't being called when the cursor moves but to Invalidation events. It looks like hit testing is still going on?

<Window x:Class="WpfApplication4.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfApplication4"
        Title="MainWindow" Height="800" Width="1024" WindowStartupLocation="CenterScreen">

    <local:UserControl1/>

</Window>


<UserControl x:Class="WpfApplication4.UserControl1"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             IsEnabled="False" IsHitTestVisible="False">
    <Line Name="m_line" Stroke="Black"/>
</UserControl>

 public partial class UserControl1 : UserControl
    {
        public UserControl1()
        {
            InitializeComponent();

            Application.Current.MainWindow.PreviewMouseMove += new MouseEventHandler(MainWindow_PreviewMouseMove);
        }

        void MainWindow_PreviewMouseMove(object sender, MouseEventArgs e)
        {
            m_line.X1 = m_line.X2 = Mouse.GetPosition(this).X;
            m_line.Y1 = 0;
            m_line.Y2 = ActualHeight; 
        }

        protected override void OnRender(DrawingContext drawingContext)
        {
            base.OnRender(drawingContext);

            drawingContext.DrawRectangle(Brushes.Green, null, new Rect(0, 0, ActualWidth, ActualHeight));

            Pen p = new Pen(Brushes.Black, 1);

            StreamGeometry streamGeometry = new StreamGeometry();
            using (StreamGeometryContext geometryContext = streamGeometry.Open())
            {
                double numPointsMul = 1;
                int count = (int)(ActualWidth * numPointsMul);
                for (int i = 0; i < count; ++i)
                {
                    double y = ActualHeight / 2 + 20 * Math.Sin(i * 0.1);

                    if (i == 0)
                    {
                        geometryContext.BeginFigure(new Point(i , y), true, true);
                    }
                    else
                    {
                        geometryContext.LineTo(new Point(i, y), true, true);
                    }
                }
            }

            drawingContext.DrawGeometry(Brushes.Red, p, streamGeometry);
        }
    }