Saxon

Saxon has built-in knowledge of the schema for the XML namespace http://www.w3.org/XML/1998/namespace, but if a schemaLocation is given in an xs:import for this namespace, the schema is fetched from the W3C web site before it is recognised as being already known. The retrieval from W3C is extremely slow because access is deliberately throttled to reduce load on W3Cs servers.

Note there's a question-mark over what we should do if a non-standard schemaLocation is provided for this namespace: should we use our local copy, or should we go to the given location (which might have different definitions). In general, if a user tries to supply a non-standard schema for the XML namespace, the consequences are unpredictable, so we should probably use our own copy regardless.