The 4S was a real landmark in UI smoothness as well, as it was the first dual-core iPhone.
Previously, even if a developer was on his best behavior (putting expensive tasks on a background thread to avoid bogging down the main thread and causing a frame rate drop), it didn't really help, because you only had one hardware thread.
Even though the 4S is a "slow" processor compared to the latest phones, it's UI was way more usable than the 4 and 3GS because of the second processor core.
Previously, even if a developer was on his best behavior (putting expensive tasks on a background thread to avoid bogging down the main thread and causing a frame rate drop), it didn't really help, because you only had one hardware thread.
Even though the 4S is a "slow" processor compared to the latest phones, it's UI was way more usable than the 4 and 3GS because of the second processor core.