TEIID-5233 highlighted a couple of issues with xml and json:
XMLEventStreamReader is very inefficient in dealing with a large number of attributes/namespaces as the iterator was traversed on each access.
JsonToXmlContentHandler was not strictly correct in the events that it emitted. Namespace and attribute events should be associated with their respective start/end element events. While the JRE libraries are tolerant to emitting the events separately, Saxon was not. To prevent issues with embedded consumers reading directly from the stax event stream this should be corrected.