How often OS/network/context overhead is the bottleneck? In my experience most of the time it's the DB. Even if it fits in RAM, complex queries always take most of the time. (Web dev here).
Some hybrid stores like MongoDB, RethinkDB and others offer more characteristics similar a traditional SQL RDBMS, while offering horizontal scaling. It's when you have several join operations that performance really takes a hit under significant load. You can't really scale a relational database in the same way.
That said, as much as I like non-relational databases, they aren't the best fit for every use case. Beyond this, most situations don't need that type of performance scaling.