Bumped into a few blogs talking about how Nokogiri was faster than REXML for Ruby XML parsing. My OMG SysML XMI to ISO STEP AP233 XML converter demonstrations are taking so long that I can't run them live during the demo - up to 8 minutes for a large example SysML diagram. So, I'm modifying the converter to use Nokogiri in the hopes that I'll be able to do the demos live in the future. My first small test showed an 80 percent improvement ... fingers crossed that holds for the real converter.
Update - having problem with XML namespaces. Only xpath method seems to understand them, so after converting from REXML to Nokogiri my converter actually takes longer to run.