I'd argue that it would as efficient is it gets if you use the database properly. You might be able to squeeze more performance out of something "thinner" like

I'd argue that it would as efficient is it gets if you use the database properly. You might be able to squeeze more performance out of something "thinner" like one of the key-value db's out there, but sooner or later you will need to scale horizontally. This is where something like Cassandra is more attractive, but it lacks a lot of typical db features, e.g. aggreagations.