Bass_X as far as I know, the ipad 3's CPU is comparable with the iPhone 4 and its gpu is faster than the iPhone 4, so if we compare the iPhone 4 and 4s, it's known that the iPhone 4s's CPU is twice as fast and its gpu is seven times as fast. The iPhone 4 is a very weak device.
On an iPhone 4s you can only redraw the entire screen about three times. Less if the pixels are partially transparent instead of fully opaque or fully transparent. You could easily hammer performance with 5 transparent fullscreen sprites on that device and the iPhone 4 is far weaker.
Also, what operating system is it running? There was a 2000% speed increase with iOS 5, and more increases with 6 and 7.
No computer in the world, even when coded natively, is powerful enough to be immune to fps dips if you throw more at it than it can handle. The truth of the matter, though, is HTML 5 is not as fast as native, and the iPhone 4s is the first device to realistically be powerful enough to make up for it, and as such, the iPhone 4s is where HTML 5 games should start targeting. I mean, maybe you could make something crazy simple for the iPhone 4, but it would be terribly limiting.