If you do frequent or repetetive transfoms of XML content, you should look into translets.
I've noticed dramatic performance improvements using translets for some forms of content (haven't collected exact metrics, though). The DocBook XSL stylesheets, along with a complex document are the best way to stress test translet performance vs. your favorite XSLT parser. (I like Saxon for most of my transforms).
Details on XSLTC translets can be found at: http://xml.apache.org/xalan-j/xsltc_usage.html
As an alternative, Jacek Ambroziak has written an XSL transformation accelerator called Gregor. Information on Gregor can be found at: http://www.ambrosoft.com/gregor.html
I'd be interested to hear from anyone who has tried either of these on complex documents, and your opinion on their performance!
See also: DocBook