Cotton gin

Basically, a machine that works like stamp mill, but instead of crushing rocks it cleans cotton. IMO cleaning cotton breaks up the pace of the game because you're just sitting there staring at your character most of the time, but you still have to click a button every once in a while. This machine could replace this boring time sink with investment in infrastructure - as they'd still clean it fairly slowly, the more fields you have the more gins you need. And while you're waiting for them finish their work you can do other things that are better paced, like preparing fields.