Strange gaming performance in Monster Hunter 3rd HD

[Bigpet]

so I do not think that this is a problem with the performance of my phone and it should normally run the game in unbuffred mode

Well not a problem with your phones GPU at least. I mean you're on a non-jail-broken IPhone which I think means that you can't use the JIT. But I can't tell you whether that's the normal performance for that OS+processor combination, since I don't really have any experience with it.