WPF: Why does text and elements blur if I use dropshadow effect on a parent item

If I add a DropShadowEffect to an parent element the text of the child elements are blurred. Why?

<Grid>
    <Grid.Effect>
        <DropShadowEffect />
    </Grid.Effect>
    <Grid.ColumnDefinitions>
        <ColumnDefinition />
    </Grid.ColumnDefinitions>
    <Grid.RowDefinitions>
        <RowDefinition />
    </Grid.RowDefinitions>
    <TextBlock Background="White">Test</TextBlock>
</Grid>

Update:

with shadow

without shadow