WPF window takes ages to render

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public OverlayView(OverlayViewModel vm)
{
  InitializeComponent();
  DataContext = vm;

  SourceInitialized += OnSourceInitialized;
  vm.PropertyChanged += OnViewModelPropertyChanged;
}

private void OnViewModelPropertyChanged(
  object? sender,
  PropertyChangedEventArgs e
)
{
  {
    if (e.PropertyName == nameof(OverlayViewModel.ShouldBeVisible))
    {
      OverlayViewModel vm = (OverlayViewModel)DataContext!;

      {
        if (vm.ShouldBeVisible)
          FadeIn();
        else
          FadeOut();
      }
    }
  }
}

  private void FadeIn()
  {
    if (!IsVisible)
    {
      Show();
    }

    // Only fade in if it's fading out
    if (_isFading)
    {
      Opacity = 0;

      DoubleAnimation animation = new()
      {
        From = Opacity,
        To = 1,
        Duration = _fadeDuration,
        EasingFunction = new QuadraticEase(),
      };

      BeginAnimation(OpacityProperty, animation);
      animation.Completed += (_, _) =>
      {
        _isFading = false;
      };
    }
    else
    {
      Opacity = 1;
    }
  }

  private void FadeOut()
  {
    _isFading = true;

    DoubleAnimation animation = new()
    {
      From = 1,
      To = 0,
      Duration = _fadeDuration,
      EasingFunction = new QuadraticEase(),
    };

    animation.Completed += (_, _) =>
    {
      Hide();
      _isFading = false;
    };

    BeginAnimation(OpacityProperty, animation);
  }