A good 10 years ago or so I was running a solution that used RDF Quad Stores - and the best one at the time (after trialling 4Store, Marklogic and some others I can't remember) was OpenLink Virtuoso - how they managed to fit a performant distributed Quad store into what started life as an SQL engine was impressive.

I've left that world now, but if you're in the market for a graph store again, it might be something to look at.