I've noticed that sometimes, specially for "complicated" structures like: https://pinshape.com/items/7557-3d-printed-skull-lamps-voronoi-style Simplify3D goes into a "not responding state for up to several hours before eventually doing the job. I've reproduced the problem both in OSX ...
I can confirm this is an Issue on 3.1, especially in complicated prints with multiple overhangs, I'm running Mac OSX 10.11.6 and I've replicated the error in win7 and win10.