Haven’t had a chance to play with it yet but I expect this is going to help track down a bunch of the TTFB issues that pop up here in the forums: Query Monitor – WordPress plugin | WordPress.org
It reports the time spent for database calls and external http requests (back-end) by the plugin that is making the calls.
For the sites that are on shared hosting and can’t install something like NewRelic it has the potential to be a life-saver (though may still require some experience to understand the results).