Ally,
That's a parameter I would hate to have enforced. Slow play will happen, regardless. I played in a 4-some yesterday and we all had trouble, it seemed, individually and consecutively. As a result, someone was always struggling and holding up the group, and the ones behind, as a result. We all had scores between 78 and 90, so bad numbers weren't the cause. If you imagine that this happens to at least 1-5 groups on a given course, per day, slow play can happen quite naturally.