Sun shadows are controled by two variables. Seems like Beth put update delay so that shadows do not flicker too often. In case anyone want to change behavior of sun shadows insert these two lines in Skyrim.ini under [Display] section:

fSunShadowUpdateTime=1.0000fSunUpdateThreshold=0.5000

These are the default values. fSunShadowUpdateTime is time in seconds it takes to animate shadow transition while fSunUpdateThreshold is probably some kind of game time multiplier which controlls the delay between shadow position updates. To make real time shadows simply set fSunUpdateThreshold to 0. You can also increase the value to make shadows not update so often, in which case you might want to increase fSunShadowUpdateTime accordingly to get slower transitions. Just remember it is shadow movement that produce flickering.

Imo real time shadows make things look worse, because then the flicker is a little bit constantly. Default settings are much better; less annoying to watch.