i hope i did understand your question correctly :)
exit_screen signal will be fired if the object is exits the screen. usefull if the object has a script attached to it that you dont need when not visible. you can then set processing false.
if you use hide() objects will not get rendered and thus your fps will go up. but your object will still be in the memory. if your world is huge (memorywise) show/hide will not be sufficent.
to save memory you could seperate your map into chunks and load only the surrounding chunks into the memory. im not 100% sure but i think godot does not render stuff that is offscreen. if im right you dont need hide/show for performance.
additionally you can try to implement LOD via the options Range begin and Range end. its pretty easy to do but you need multiple LOD models