It is a tool to search for the application which is causing the "user accesibility" setting can not be changed.
The function is very simple just find the application using TYPE_SYSTEM_WINDOW and display the application details page.

If "Draw over other Apps" is displayed while setting application is running some settings can not be changed for Android security reasons.
This application is "Draw over other Apps"
Get and display a list of. When you tap on the application it will change to the details screen of the application so press the Force Stop button and change the settings that could not be changed.

So it does not work in the background it is not the cause of slow operation.