XML::Rules might be slightly quicker than XML::Simple, because it doesn't try to guess what would be the best way to convert the XML into a data structure, but that's not the main reason you should switch. The problem with XML::Simple is that (even if you do specify some options) the datastructure it generates is not consistent. Things like optional attributes and repeated tags may cause hard-to-handle inconsistencies. Well set XML::Rules will give you a consistent datastructure and allow you to ignore and skip data you are not interested in. This may lessen the memory footprint and speed things up.
Jenda Enoch was right!
Enjoy the last years of Rome.