Add PerfBar to your website! http://lafikl.github.io/perfBar/
It's a neat little JS script that adds a bar of performance metrics to the bottom of your screen.
Only default options right now.
I am working on adding an admin form where you can control which stats show up in the bottom, and an alter to add your own stats, but I'm trying to keep it done in a performant way. The default options are pretty good anyway, as all of the stats on there are useful.