When simulating on Xcode I can see that the frame rate is about 60fps. I also know that the human eye can see between 10-12 frames in a second and define them.
Let's say I would like to replace an image on the iPhone, as fast as I can, for example running all the numbers images again and again. What will be the max speed that can be produced without any lags (that my eye can't see), and what would happen if I put it to be faster than the max? Will it just jump over some numbers?
What does it means when I see 60fps? Why not 30? Can we see the difference?
Are there any docs or specs to understand the exact screen spec?
The iPhone 4s, iPhone 5, and newer devices can update the screen at 60 frames per second. The iPhone 4 and older devices can update at 30 fps. If your code updates each frame faster than the device can update the screen, it will simply skip frames.
As for whether fps is noticeable, that depends on the person. I cannot stand a framerate lower than 30 and I can clearly tell the difference between 30 and 60. Some people can't tell the difference. Some people prefer 30 to 60. It really depends on who you ask. But in general, the higher the framerate, the smoother the animations will be.