The XSL Cache extension is a modification of PHP's standard XSL extension
that caches the parsed XSL stylesheet representation between sessions for
2.5x boost in performance for sites that repeatedly apply the same
transform. 

Although there is still some further work that could be done on
the extension, this code is already proving beneficial in production use for
a few applications on the New York Times' website.

WWW: http://code.nytimes.com/projects/xslcache/