What does idle time mean?
Idle time refers to the time spent in scripts' _process() methods, in addition to rendering and Godot's internal CPU usage (everything that's not physics-related).
When the idle-time is too high, then your _process() is waiting for something to finish.. maybe vsync or any other timer-consuming function.
This video will definitely helps you.
This is not a video!
Nevermind. Me being blind.