ok, i don't seem to be able to reproduce it with phonegap export, but in appmobi export, how to reproduce:
1) create a 500x500 jpeg background with some image, scale it to 100x100
2) create a 500x500 sprite with some opacity, scale it to 100x100
3) add a rotating behavior to the sprite with some medium speed of your choice
4) export through appmobi and install
5) run on device
6) double tap back button to exit (put app in background - you can see in application manager that the app is still running in background)
7) run app again
8) repeat steps 6 -> 7 to see fps degradation of the rotationg sprite