How do I make objects load on entering window?

I have a big layout with hundreds of objects that move. I want them to only load when the player gets near them. How can I do this?

Alternatively, will it still affect performance if I just disable the object's ability to move when they are out of vision?

With hundreds of objects you should definitely disable behaviors for objects which are too far away. You can use "Is on screen" condition or distance() expression. So you can do something like this:

Every 2 seconds:
 Pick EnemiesFamily where distance(EnemiesFamily.x, EnemiesFamily.y, Player.x, Player.y)<2000
	EnemiesFamily enable behaviors

 Pick EnemiesFamily where distance(EnemiesFamily.x, EnemiesFamily.y, Player.x, Player.y)>2000
	EnemiesFamily disable behaviors