Problem with BG image and performance

Good day everyone.
I’ve made a game with parallax backround images in it.
Background consists from four different moving layers. Without bg I have 60fps on iPhone4. But when I add it on the stage fps is reduced to 45-50.
I divided bg images into tiles 128x128. Problem still exists. All images in png extension with alpha channel. I’m using the last version of cocos2-x. Have anybody an idea why fps reduced so much?