- Open devtools
- Select Profiles
- Click Start
- Do the (typically slow) things you wish to record
- Click Stop
You now got a table of functions, and CPU time they consumed. On the right hand side of the table, you got a link to the js file and its line number of the function that is responsible for this. You can click them to open the files in the Sources tab.
Check out this chapter in CodeSchools awesome Devtools lesson for more on this topic: http://discover-devtools.codeschool.com/chapters/6/