Sorry if this is a bit late for a reply, but I found that numerous things can cause game slowdown:
-Too many animated objects
-Too many sounds playing at once
-Large sprites reduced in scale
-Layouts too big
-Faded interface buttons
I used to be rather frustrated with Cocoonjs myself, not sure if they snuck in an update but my soon to be released mobile game has a far better frame rate when I eliminated these things above. And my phone is pretty ancient (2.3.5 version Android).