But if you set objects Opacity to 0, the engine will still render this object on the screen but invisible.
Second thing, objects outside the visible screen as I remember, are automatically not rendered by the engine.
It probably depends on the platform, browser or WebGL version.
Try setting Text object opacity to 0 and then changing its text - TextWidth and TextHeight properties are not updated, because the text is not actually rendered. If you set opacity to 0.01, it starts rendering.
Off-screen objects are not rendered, but they still consume CPU resources if they have active animations, behaviors, or they are part of different events, collision checks etc.