http://gm.zoomlab.it/2014/xhprof-varying-vagrant-vagrants/

Sometimes you need to profile things. This tutorial helps.

(Update – Jan 3, 2024: This old link was super helpful to me when I started using XHProf in 2016 but it no longer exists. Internet Archive to the rescue. That said, a more current approach to setting this all up now exists in the WordPress Performance Team handbook).