GNU JAXP 1.3
EHR points to GNU JAXP 1.3, a JAXP implementation that — among a lot of other features — uses JNI to wrap libxml2 and libxslt. Given that these are considered the fastest XML parsing and transformation implementations in existence, it might be interesting to try them out where a 100% pure Java implementation is not mandatory.
Only a test because I got complaints the response times are too long.