I got tired of the render being stopped half way through after 3:07 h of rendering so I rendered the whole scene in pieces and saved occasionally. Sometimes having too much history built up can cause renders to lagg. Be sure and delete your history as often as you can. As far as rendering a shadow pass with your enviorment; I recommend rendering those separately but thats just my preference. Rendering in separate passes is always a faster method of getting the job done faster. BTW, could you post a wire of your scene?