Hi Kingsley,
The Semantic Web community hasn't focused exclusively on query execution speed.
Let me clarify myself: the scientific SemWeb community mostly focused on speed, as is apparent from publications about SPARQL query execution (and, from personal experience, many researchers and reviewers still having trouble to understand why speed is not our main focus).
Anyone that encounters a service (Web or Semantic Web) expects results in acceptable timeframes (typically <= 250ms) , that's a function of user behavior on the Web or anywhere else.
Yes, and it is my opinion that public SPARQL endpoints overpromise in that regard. The whole public SPARQL endpoint discourse has made us believe that it is actually realistic to have free+fast+high availability, as is the case for any other Web service. But given that SPARQL is more expressive per request than any other Web service I know, this cannot hold.
In simple terms: SPARQL is a very expressive and hence very expensive API.
In technical terms: show me any other API that exposes a PSPACE-complete interface.
You will find that Wikidata, is doing the very same thing, but with much more hardware at their disposal, since they have more funding than DBpedia, at this point in time.
Indeed.
Your "Simply doesn't work on the public Web" claim is subjective
Let me clarify "simply doesn't work": companies/institutions that host their data in any other API on the Web will see a substantial increase in server costs when they try to host that same data as a public SPARQL HTTP service. My claim is that this increase is so substantial, that SPARQL endpoints cannot become a reality on the public Web at the same customer cost (= often free) of any other API on that same Web, and hence will not become a reality. Concretely, for most institutions that want to make their data queryable for free, the SPARQL protocol will simply be too expensive for their budgets. Alternatives, like dumps, LD documents, TPF, might be feasible, but they all come at another cost. No silver bullet.
So far, that claim has not been proven wrong.
Best,
Ruben