Farhad Manjoo, The New York Times:
What are the costs of upgrading? Performance. Ars Technica installed iOS 8 on the iPhone 4S and the iPad 2, and in both cases it found that apps opened up slightly more slowly under the new OS. It took about 2 seconds instead of 1 to open up Safari on a 4S running iOS 8 rather than 7.
The iPhone 4s was released in October, 2011. That is four years ago, which for a high-tech device such as a smartphone is a very long time. That the latest mobile operating system from Apple works on something that old is remarkable.
Yesterday I updated my iPhone 4s to iOS 8, and yes there is a negative impact to performance. Not surprising. But what is surprising is that my iPhone 4s is more than usable. Even though my iPhone 4s runs a little slower iOS 8 is worth the update, especially because of improved security.