It's fast because it doesn't actually save your data when you ask it to. It saves it later, when it feels like it, maybe. Also, the hate I have for Mongo is from first-hand experience with the wretched thing. The scaling is insane. The next jump after your first server is like 9 servers (2 repsets w/ 3 servers each w/ 3 config nodes). I don't have that kind of cash laying around for servers, especially when instance-for-instance, Postgres can handle easily 10x the write load of mongo.