Wpf holding modifer keys causes animation to stutter

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.NET

XAML

WPF

My code rotates a cube when Key.Q is held down. The animation looks smooth. If I press and hold Key.Q and then I press and hold Shift, Alt, Ctrl or CapsLock, the animation visibly stutters (holding Tab, Backspace or Enter does not). If I'm holding Key.Q and a modifier key and I release the modifier key the animation is smooth again. This is undesirable behavior and I want the animation to not be affected if these other keys are held.

Is this a background process being captured and if so is there an override for this behavior?

<Viewport3D Name="ViewportMain">

        <Viewport3D.Camera>
            <PerspectiveCamera x:Name="camMain" Position="6 5 4" LookDirection="-6 -5 -4">
            </PerspectiveCamera>
        </Viewport3D.Camera>

        <ModelVisual3D>
            <ModelVisual3D.Content>
                <AmbientLight Color="#ffffff" />
            </ModelVisual3D.Content>
        </ModelVisual3D>

        <ModelVisual3D x:Name="MyModel">
            <ModelVisual3D.Content>
                <GeometryModel3D>
                    <GeometryModel3D.Geometry>
                        <MeshGeometry3D x:Name="meshMain" 
                            Positions="0 0 0  1 0 0  0 1 0  1 1 0  0 0 1  1 0 1  0 1 1  1 1 1" 
                            TriangleIndices="2 3 1  2 1 0  7 1 3  7 5 1  6 5 7  6 4 5  6 2 0  2 0 4  2 7 3  2 6 7  0 1 5  0 5 4">
                        </MeshGeometry3D>
                    </GeometryModel3D.Geometry>
                    <GeometryModel3D.Material>
                        <DiffuseMaterial x:Name="matDiffuseMain">
                            <DiffuseMaterial.Brush>
                                <SolidColorBrush Color="Red"/>
                            </DiffuseMaterial.Brush>
                        </DiffuseMaterial>
                    </GeometryModel3D.Material>
                </GeometryModel3D>
            </ModelVisual3D.Content>
            <ModelVisual3D.Transform>
                <RotateTransform3D>
                    <RotateTransform3D.Rotation>
                        <AxisAngleRotation3D x:Name="rotate" Axis="0 2 0"/>
                    </RotateTransform3D.Rotation>
                </RotateTransform3D>
            </ModelVisual3D.Transform>
        </ModelVisual3D>

    </Viewport3D>

Public WithEvents Timer1 As New System.Windows.Threading.DispatcherTimer()
Public ax3d As Media3D.AxisAngleRotation3D
Public myRotateTransform As Media3D.RotateTransform3D
Public isrotating As Boolean = False

Private Sub Window_Loaded(sender As Object, e As RoutedEventArgs)

    ax3d = New Media3D.AxisAngleRotation3D(New Media3D.Vector3D(0, 2, 0), 1)
    myRotateTransform = New Media3D.RotateTransform3D(ax3d)
    MyModel.Transform = myRotateTransform

    Timer1.IsEnabled = True
    Timer1.Interval = TimeSpan.FromMilliseconds(1)
    Timer1.Start()

End Sub

Private Sub Timer1_Tick(sender As System.Object, e As System.EventArgs) Handles Timer1.Tick

    If isrotating Then

        ax3d.Angle += 4

    End If

End Sub

Private Sub Window_KeyDown(sender As Object, e As KeyEventArgs)

    If e.IsRepeat Then
        e.Handled = True
        Return
    End If

    If e.Key = Key.Q Then

        isrotating = True

        e.Handled = True

    End If

    e.Handled = True
End Sub

Private Sub Window_KeyUp(sender As Object, e As KeyEventArgs)

    If e.Key = Key.Q Then

        isrotating = False

        e.Handled = True

    End If

End Sub

What I have tried:

I've tried other keys besides Q.

I've made several attempts to capture if modifier keys are held, i.e. "If e.Key = Key.LeftShift Or e.Key = Key.RightShift Then..." to capture the keydown and handle and exit the event but this hasn't made any noticeable difference.