It's all in the CPU/GPU. The iPhone 4S is simply as fast as it can get, both for the system reactivity and the gaming performance. As long as the screen retains the current resolution, there is no point in putting an upgraded CPU in it. For this reason, Apple is going to put a bigger screen, with an higher resolution, in the next iPhone. The 3GS is going to get discontinued with iOS 6, so Apple can afford to introduce another resolution in the ecosystem. The most obvious one would be 1920 x 1280, a 2x scaling of the current one. But making screens with such pixel density could be near impossible. Even at 4.5", the screen would be 512.79 PPI, and even though Toshiba demoed 494 PPI displays last October, we don't know how viable they would be in mass production. Moreover, even if Apple manages to get 500ppi screens in mass production, and the A5X is certainly up to scratch for driving it, we don't know how the battery and heating (seen in iPad 3gen) would be addressed. The thing is, either Apple manages to address all of these issues, or you can expect the next iPhone to be pretty much a 4S plus LTE and the extra battery to take care of that. There is no point in upgrading the A5 unless the resolution, along with obviously the screen size, gets boosted.