How much actual computation is it really though? Especially compared to all the other computations happening hundreds of times per second to render the display

How much actual computation is it really though? Especially compared to all the other computations happening hundreds of times per second to render the display at all? Also given that the shadow is part of the pen interaction, they're almost certainly already calculating most of what you would need to render that shadow in the first place just to handle the tilt sensitivity.