I have tried different method to see if there any improvement. 1 st I have about 60 objects on a level(not on screen), the framerate is about 20 to 30 fps. but it will get better after a while. or after I restart the game several times.
Then I also tried remove most of the enemy objects, spawn them by certain event(e.g. re-spawn enemy). then the framerate is better, about 30 to 35fps.
So here is my conclusion: Existing object will eat performance even they are not draw on screen. If I have a big level with pre-placed enemies, how do I improve performance?