There's some mix between "it's slow" and "it sets its timeout threshold too low" - a lot of queries would be OK if they just had a bit more time to run. And unfortunately, the time wasted on the badput of the killed queries just slows down everyone else. (They really need a batch queue)

The Wikidata folks are well aware of the limits on their SPARQL service. They just posted an update the other day:

https://lists.wikimedia.org/hyperkitty/list/wikidata@lists.w...

seems to depend on the query. I can issue straight forward queries that visit a few hundred thousand triples easily. But when i write a query that visits tens of millions of triples it times out.