What is the optimization for using unmovable backgrounds on rendertextures?
I don't really understand what you want to know here (what is the optimization?
unmovable backgrounds?
), but I hope my answer will go in the right direction...
The use of a RenderTexture is advised as soon as the static content that shall be drawn gets to a complex level. Hard to say what complex means, but for instance if you have a map generator, that needs to run once to create a map, it would be stupid to generate that map every single iteration.
But on the other hand if you have a text and a rectangle, then it doesn't nessecarily make sense to render that to a RenderTexture, since there won't be any performance issue on drawing one object (the RenderTexture) or drawing two objects (the rectangle and the text).
So the decision whether you should use a RenderTexture depends on the complexity and 'staticness' of the scene.
I'd also like to propose the creation of a thread of general small questions. Making a thread for every detail I don't understand well isn't something I am fond with.
One thread for everyone or one thread for each user?
If everyone would ask questions in one thread then we'd soon have quite a mess.
If you have many different small things to ask, then you might want to join the
unofficial IRC channel. Many people hang around there and are happy to answer some of your questions.