Currently, we have a custom XML serialization module at lib/MusicBrainz/XML.pm which creates blessed objects for every element, then serializes the entire tree recursively. Profiling I've done has shown that switching to XML::LibXML (which, as the name suggests, uses libxml2) would improve performance and reduce memory usage overall, most likely because it avoids overhead related to the number of Perl objects being created in our current implementation.