If you're using Python to serve a website using Flask or Django, you need to take a look at:

https://github.com/phleet/flask_debugtoolbar_lineprofilerpan... (includes screenshot)

or https://github.com/dmclain/django-debug-toolbar-line-profile...

It provides a very intuitive way to figure out where, exactly, your view is slow. I know the Chrome and Firefox teams are both hard at work on building more robust profiling tools, but I would love to see them integrate something for JavaScript that's this easily accessible.