I havent had it happen much. I suspect he had it parked to the system bar and doing something else in the foreground.
One thing I think is interesting is that the processing keeps up with the game play. When it takes me an hour to do my turn, then it takes maybe 2 minutes to process it. Just long enough to make me walk away and see what I missed during the Last hour.
If it took hours to do the moves and less than a minute to proccess, Id die of starvation or dehydration or exploded bladder. (this is a dangerously addictive game)