A team I've been working with for years has had a lot of success with Blazegraph: https://www.blazegraph.com/ -- the marketing materials call it "ultra-scalable," and that actually turns out to be true. I haven't seen too many cases where specific queries will cause serious performance with the system.

That said, we've done some work to prevent runaway queries (e.g. strict query timeouts, downstream systems that handle that situation gracefully).