post a .capx to see what go wrong !!!
by my experience if you have more than 50 objects (sprites) the game will go runing slow but in your case its 40 objects (and they are small sized as you say) so post .capx maybe i can optimize it for you
remember to
- set clear background to NO
- every layer to transparent
- avoid every tick actions (thats important)
- enable / desable collisions only when it is necessaire
- enable web gl should optimize render cpu and memory but only on faster device (iphone 4s and more , samsung s3 and more)
- avoid inuseless physics and actions
- use tiled background when it is necessaire
thats all wgat i can recommand to you but better is to see the .capx to see what go wrong