@a6ff0b2b I dropped it to 302 windows and the lag is there, but the jumpiness is greatly reduced.  I moved it up to 325 windows and it's now incredibly jumpy (as is the Safari window).  A second user definitely seems to lower the threshold. And attempting to drag windows in this state causes an extreme CPU spike. 

 @731bb49a Hold down Option when dragging to remove the jumpiness, leaving only the lag. (Option disables the OS-level window snapping.)