Christian Stocker points out an interesting contribution by the web developers over at the New York Times website - the xslcaching extension for PHP.

It caches the parsed XSLT stylesheets into your apache child memory (shared memory is on the todo list) and reuses it at the next request. [...] As - depending on the size of your stylesheets - the importing of XSLT stylesheets may take a considerable amount of time, this extension may really improve your website's performance.

Check out the project's page for full details on the installation, use and license for it (including some links for further reading).