Diagnosis beta uses small overlay to tell you what’s happening
Diagnosis by developer Dark3n is a task manager that allows you to see what is going on with your foreground application without pausing and putting it in the background like other task managers do. It works by placing a small configurable text bar over your app and updating it every few seconds.
You can configure it to view CPU usage, data transferred, bytes written, active applications, free memory, top CPU hogs, processor speed, battery temp, and many other things. It can also log information to a database for later viewing.
Diagnosis is in open beta at the moment, meaning you should expect bugs and issues, but if you're trying to figure out what app you downloaded over Christmas is causing you major grief, this can help identify why your phone went from blazing fast to amazing trash in under a week. It also does not require root so it should play fine with stock phones.[xda-developers]