I agree with your sentiment, it would be great if Rhino could implement a lightweight, fast vector sun shadow in the display port.
My workaround, which is not as good, has been to dig into the display options and use the below script to change how much memory Rhino (6) uses for rendering shadows. This works best on good hardware. Have a play with the slider settings under shadow, and make sure the scene lighting is set to ‘sun’ - that should improve things a little anyway.