Well, not the performance problem. So many mines appear in so many systems that the entire game slows down. If you actually go into one of them, I get a slideshow. A minesweeper won't solve that, because it'd still take 45 minutes to clear the field at slideshow speeds, after the other defenses are cleared.
As it stands now, once the Vasari mine spam starts Entrenchment is unplayably slow on my system. Original Sins ran at high settings even on the biggest maps without problems. A minesweeper won't solve that.