In which Aaron finds and fixes slow spots in Bundler and ExecJS and makes it look easy.
http://tenderlovemaking.com/2011/12/05/profiling-rails-startup-with-dtrace/
In a side-note, Aaron references a technique from "Working Effectively with Legacy Code", which looks like a good read.
http://www.amazon.com/Working-Effectively-Legacy-Michael-Feathers/dp/0131177052