Not exactly 1 second behind, It would at most be 1/30th of a second behind (if the games is at 60fps). A delay of 1/30th of a second would be hardly noticeable, in comparison to the halving of the time needed to process shadows.
I am trying to figure out an efficient way of using shadows for waving trees (of which there are many)..
Maybe I could render the animated shadows from a top view in blender, then save them as an animated sprite. I could maybe then use the transparent sprite as a shadow..
I've seen games with many thousands of trees, all waving, with animated shadows (e.g AOE III). I wonder how they would do it..