The gotcha here is that the selectors in that example SASS file are pretty inefficient. In that screenshot, you'll end up with a selector like ".main .site_box p .name", which the browser is going to parse from right to left. By putting everything behind a class namespace like that you force a potentially unnecessary class lookup on every selector. When you factor in how far nested SASS lets you get in combination with how much CSS a large webapp will require, it can have a large impact on page load speed.