Federated Queries with Wikidata are very slow

[TTG3333]

I tried the following command, which I think is rather simple, and it took 137 seconds to run. In comparison, the same command without the SERVICE or FILTER parts takes 33.9 seconds to run.

SELECT ?archive ?archiveName ?archiveID ?archiveInception
WHERE {
  GRAPH <https://www.diamm.ac.uk/> {
    ?archive wdt:P2888 ?archiveID .
    ?archive rdfs:label ?archiveName .
  FILTER (STRSTARTS(STR(?archive), "https://www.diamm.ac.uk/archives/"))
  }

  SERVICE <https://query.wikidata.org/sparql> {
    ?archiveID wdt:P571 ?archiveInception .
  }

  FILTER (?archiveInception >= "1900-01-01T00:00:00Z"^^xsd:dateTime)
}

Such delays means that any queries that are minimally more complex will take a very long time to run, and this is something we should look into seeing how we can speed it up.