Snow Leopard's iteration of Mail has a weird above-below layout that provides too little vertical space for reading messages: (This layout was common in many Mac apps at the time, for some reason.) There were multiple times in High School when I lost many hours of work on my Snow Leopard machine, because when I get 'in the zone' of writing, I stop remembering to hit cmd+S. In a VM using both side-by-side, 10.9.5 feels snappier, both in general and under resource-constrained scenarios, because in the latter case Mavericks uses memory compression.
