I've just read this post at CopyBlogger which makes a compelling case for making sure that your web pages load as quickly as possible.
Load speed is a signal used by Google. But just as importantly, very small delays in load speed loses visitors.
Reactivity is free when downloaded from the web and tests your page, breaking it down and showing all the files it finds with load times for each (using a single thread for a more accurate measurement of each).
You'll easily spot bottlenecks - often third-party code or plugins can cause such delays and be difficult to track down.
Reactivity is a standalone component of Scrutiny which is a suite of website analysis tools.