If you don't need up to the second freshness, I'd recommend using QLever instead of WDQS. It returns a couple million results (which seems low, but I didn't double check your query) in a second or two. They are currently running the dump version 17.07.2025

https://qlever.cs.uni-freiburg.de/wikidata/OFP4AK

https://qlever.cs.uni-freiburg.de/api/wikidata?query=PREFIX+schema%3A+%3Chttp%3A%2F%2Fschema.org%2F%3E%0APREFIX+wd%3A+%3Chttp%3A%2F%2Fwww.wikidata.org%2Fentity%2F%3E%0APREFIX+wdt%3A+%3Chttp%3A%2F%2Fwww.wikidata.org%2Fprop%2Fdirect%2F%3E%0ASELECT+%3Fpage_title%0AWHERE+%7B%0A++++++++%3Fitem+wdt%3AP31+wd%3AQ5.%0A++++++++%3Farticle+schema%3Aabout+%3Fitem%3B%0A+++++++++++++++++schema%3AisPartOf+%3Chttps%3A%2F%2Fen.wikipedia.org%2F%3E%3B%0A+++++++++++++++++schema%3Aname+%3Fpage_title+.%0A%7D