Ios app framerate drops down if restarted after termination
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i'm making a puzzle game for iPhone and recently discovered a weird behaviour.
I'm building against ios 5 and testing on an iPhone 4 with updated ios 5, the game is entirely built using CALayers, i'm not using external frameworks like Cocos2d, just Apple frameworks (quartz, twitter, avaudio, ipod). It's a simple game, sort of a Colums / PuyoPuyo / Bejeweled.
At the first launch the game is able to keep a framerate of 120fps, pretty constant, sprite movement is extremely smooth, everything works fine. If the game goes to background and then is recalled everything is ok and this is the usual behaviour.
There is one special case: if the game goes to background and then i decide to terminate it brutally from the bar and then launch the game again immediately after termination, often but not always, the framerate goes down to 70-80, and everything is a bit laggy; this does not improve with time, it's not just slow at startup, it remains constant at this rate and remains a bit laggy.
One thing i observed (but could prove wrong or misleading) is that if after i terminate the app i await some time before opening it again the problem doesn't happen.
The problem is the same with one or fifty sprites on screen, it's not related with that.
Of course there are no other apps in background, the game is the only one running.
The game never crashes on its own, in instruments it looks like there are no leaks and memory usage is a little more than 1mb of memory in the worst scenario.
I can't really understand the possible reason for such a problem, this makes no sense to me.
I never experienced something like that but i'm pretty new to ios development, i've created several apps, but this is the first one i'm creating for publishing on the app store.
At first i thought the problem could depend from some values read from the userdefaults at the startup, but even if i disable loading the preferences (so that the app always uses the same default values at startup) this behavior is still there.
Tested on ipad 1st gen with updated ios 5 gives the same results but since the hardware is faster i tested at 240fps to verify the problem, on ipad the framerate drops from 240 to 180-190.
The problem is not that the framerate is too slow since the game will probably run at 60 fps in the end but this behaviour is so strange, such a difference in performance is not acceptable; i guess there is something wrong in my code, maybe something i should do when the app gets terminated? I could not figure out what could it be. Any suggestion? Thank you.