FPS drops great when Layer zoom in

I have a very big CCLayer(8640px * 5760 px), and i scale the layer when zoom in. The normal FPS is 30 with no scale. But when i scale the layer to 1/3, the FPS drops to 12, then i scale the layer to 1/9, the FPS goes back to about 15.
I don’t want the FPS drops too great when zoom in. Could anyone give me some suggestion?

Debugged on iTouch 4 with cocos2d-x 0.12.0