This probably exists elsewhere, but I feel someone might find this useful. In my Deity Huge histographic game, after I had everything irrigated and railroaded, I managed to get things down to about 2 minutes per turn, if even that. The key lay in not having to re-activate workers every turn, and I actually discovered it by mistake. First you clean all the pollution you got from the inter-turn. Don't fortify the workers, keep them active. Then click on the circled button in the upper left corner of the square at the lower right corner of your screen (see screenshot below). You'll go to the next turn and your workers will all have a turn.
