WPF animation drawing performance when collapsed

I have a loading indicator that looks like this:

Now I made it spin using a storyboard and it works fine. As far as I understood the animation in WPF, once the visual is collapsed/hidden the animation does not take any more resources (eg. the spinning).

But I realized, that even when it's collapsed it's still using a lot CPU power. Here is the code:

public partial class MainWindow : Window
{
    public MainWindow()
    {
        DataContext = new Vm();
        InitializeComponent();

    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        Spinner.Visibility = Visibility.Collapsed;
    }
}

internal class Vm
{
    public Vm()
    {
        for (int i = 0; i < 5000; i++)
        {
            Items.Add(new Item());
        }
    }

    public List<Item> Items { get; } = new List<Item>();
}

public class Item { }

XAML:

<Window x:Class="test.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:test"
    xmlns:PresentationOptions="http://schemas.microsoft.com/winfx/2006/xaml/presentation/options"
    mc:Ignorable="d"
    Title="MainWindow" Height="450" Width="800">
<Window.Resources>
    <SolidColorBrush x:Key="THEME_LoadingSpinnerBackgroundBrush"
                 Color="#ebf0f5"
                 PresentationOptions:Freeze="True"/>
    <LinearGradientBrush x:Key="THEME_LoadingSpinnerForegroundBrush"
                     PresentationOptions:Freeze="True"
                     StartPoint="0,0"
                     EndPoint="1,0">
        <GradientStop Color="Red"   Offset="0"/>
        <GradientStop Color="White" Offset="0.5"/>
        <GradientStop Color="#000f789b" Offset="0.9"/>
    </LinearGradientBrush>

    <Storyboard x:Key="LoadingIndicatorStoryBoard"
            PresentationOptions:Freeze="True">
        <DoubleAnimation Storyboard.TargetName="icon"
                         Storyboard.TargetProperty="(Canvas.RenderTransform).(RotateTransform.Angle)"
                         From="0"
                         To="360"
                         Duration="00:00:1.3"
                         RepeatBehavior="Forever" />
    </Storyboard>

    <Style TargetType="{x:Type local:LoadingIndicator}">
        <Setter Property="Focusable"
            Value="False"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type local:LoadingIndicator}">
                    <Grid Width="20"
                          Height="20">
                        <Canvas x:Name="icon">
                            <Ellipse Canvas.Left="0"
                                     Canvas.Top="0"
                                     Width="20"
                                     Height="20"
                                     StrokeThickness="2"
                                     Stroke="{StaticResource THEME_LoadingSpinnerBackgroundBrush}"
                                     StrokeMiterLimit="4"/>
                            <Ellipse Canvas.Left="0"
                                     Canvas.Top="0"
                                     Width="20"
                                     Height="20"
                                     StrokeThickness="2"
                                     Stroke="{StaticResource THEME_LoadingSpinnerForegroundBrush}"
                                     StrokeMiterLimit="4"
                                     StrokeDashArray="17 25"/>
                            <Canvas.RenderTransform>
                                <RotateTransform CenterX="10"
                                                 CenterY="10"/>
                            </Canvas.RenderTransform>
                        </Canvas>
                        <Grid.Triggers>
                            <EventTrigger RoutedEvent="Loaded">
                                <BeginStoryboard Storyboard="{StaticResource LoadingIndicatorStoryBoard}"/>
                            </EventTrigger>
                        </Grid.Triggers>
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

</Window.Resources>
<DockPanel>
    <Button DockPanel.Dock="Top" 
            Content="hide"
            Click="Button_Click"></Button>
    <ItemsControl ItemsSource="{Binding Items}"
                  Name="Spinner">
        <ItemsControl.ItemTemplate>
            <DataTemplate DataType="{x:Type local:Item}">
                <local:LoadingIndicator />
            </DataTemplate>
        </ItemsControl.ItemTemplate>
        <ItemsControl.ItemsPanel>
            <ItemsPanelTemplate>
                <WrapPanel />
            </ItemsPanelTemplate>
        </ItemsControl.ItemsPanel>
    </ItemsControl>
</DockPanel>

CPU is at 10% on an almost empty window:

Can anyone tell me why the CPU usage won't go down?