Both Cayley and TitanDB aren't native graph databases. In fact, Cayley supports many storage engines, including MongoDB etc. This is because both are graph layers, and the data maintenance is done via the real database underneath. This has benefits in the sense that it's easier to build, but also doesn't perform as well when it comes to query latency.

DGraph, OTOH, is a native graph database. We do use RocksDB, but the data distribution and maintenance is done by us. It's optimized for decreasing the number of network calls, to keep them linear to the complexity of the query, not the number of results. This is of incredible value when you're running real time queries, serving results directly to end user. The query latency hence, isn't too affected by high fan out of intermediate results and should remain low; while providing high throughput.

In fact, the entire HN traffic is being served by one GCE n1-standard-4 instance right now, using all 4 cores really well :-).