Thanks for lots of helping!
@BluePhaze: my capx is kinda of complex now and I used several custom plugin/behaviors in it so you might not able to open it:/ But I take some screenshot to explain it:
This is the layout. As you can see I have many sprites obj block out the level in stead of tiledbackground, because sprites have image point so I can use them to generate visual objects, and sprite can be scaled up so it will make some blur effect I need.
@Ashley : I have iphone 3gs on iOS 6, ipad on iOS5(6 is not supported )
@Arima : I tried your suggestion and set most objects to invisible, the framerate doesn't change too much, but a little more stable. Is there anyway to test on-screen overdraw? because I do have several overlay from foreground to background.(I even have a full-screen overlay for gradient effect)
And thanks for the suggested plugin, I will try it out:)