GDPerformanceView-Swift from Gavrilov Daniil is an open source component that allows you to display performance details of your app within the status bar.
With GDPerformanceView you can display the frames per second rate, cpu usage, app version, and the iOS version at the top of your application.
This is an image from the readme showing GDPerformanceView in action:
You can find GDPerformanceView on Github here.
A nice library for monitoring resource usage.