Large-scale, complex information integration at big companies every day. The forgotten "V" of Big Data, Variety, i.e., schema complexity, is the sweet spot of semantic technology.

It is not at all fast.

I don't even think it's computationally possible for SPARQL to be fast.

SPARQL is PSPACE-complete. Worst case complexity and "fast in practice" aren't really the same thing at all. I suspect average case complexity for SPARQL is much better, which is backed up by several reasonably peformant implementations.

What are these reasonably performant implementations? Have you tried them with a billion edges?

Also, if your data store is "fast in practice" but has worst cases that are PSPACE-complete, how do you prevent worst-case queries from DOSing it?

Well, I'm a vendor in the space so I like my implementation: http://stardog.com/ -- and yes we've "tried it" with 10s of billions of edges.

Worst cases are prevented from DOSing by having query management features like auto-killing queries that run too long, etc.