On DZone.com John Esposito reminds you about another technology Facebook has introduced to the world of PHP (besides HipHop) - XHProf, a PHP profiler.

Facebook did more for PHP: they also created XHProf, a PHP profiler with a (supposedly) easy-to-use HTML interface, designed to pinpoint exactly where your bottlenecks are appearing, so that you can optimize at every stage in the pipeline. [...] If you haven't tried XHProf, you might want to look into it. Installation apparently requires a little nudging, but Nick Lewis just posted a full, practical guide to benchmarking and performance tuning your PHP and MySQL, using XHProf (as well as other techniques) -- a very nice overview of many common bottlenecks and how to open them up.

There's also a link to some Drupal 6 benchmarks that shows how it has helped that project (including both small and large improvements).