Christian Stocker points out a handy patch for the PHP XSLT functionality today - one that allows you to test to see where in the transformation the most time is spent.
Look no further than this patch to the PHP XSL extension. It uses the built in profiler of libxslt to get the information which template was called how many times and how much time was spent in them. It may help in improving the speed of your XSLT transformations, like a PHP debugger can help in profiling your PHP scripts.
Besides the link, he also includes a simple sample script that uses the setProfiling function to write out to a text file the stats of the transformation. He also includes a (very beta) patch to the libxslt functionality itself to make it easier to not only time the overall execution of the xslt, but also allows for timing of other individual templates that might be included.