For the record, iOS 9.2.1 was released earlier and appeared have an innocuous update concerning on security and operating bug fixes. That was Apple’s official release notes stated.
A YouTube user with his compulsive iOS speed comparison discovered that iOS 9.2.1 brings a significant speed improvements to iPhone 4S and the recent iPhone 5. Both devices still lag behind its old iOS 8 in terms of boot times, but majority of the apps now open either as fast or slightly faster and scrolling is noticeably smoother.
This performance shift in particular to the secrecy behind is interesting because speed degradation of iPhone 4S from iOS 8 switching to iOS 9 is the heart of the current lawsuit that was filed to company. This potentially explains why Apple did not bring up the improvements in iOS 9.2.1 in its official release notes (of course they will admit guilt).
There seems some reason for iPhone users who doggedly sticking with iOS 8, not to give iOS 9 a try. Their disinclination now has become understandable since there is no way to downgrade iOS 9 once switch is made.