On older devices like iPad2 or iPhone4 there's always a fraction of a second that the game stops, whenever some particular sprites get shown the very first time. If that sprite gets shown the second time, it's all fine.
I don't have Unity Pro, so I couldn't use the exact profiling and now I can't even say a 100% sure that it's because of any toolkit 2d code. But generally, is there any known performance consuming code that gets run every time a sprite is shown the first time?