I see so many people here saying that their performance in older iPhones has declined with iOS 11.
I would like to point out that most (if not all) the lag issues and other performance related issues can be reproduced on a current iPhone 8. I have one. The flashlight has a noticeable delay when turning it on, the calculator is broken as described, the UI gets stuck a lot, apps crash a lot etc.
This is not a money grabbing scheme by apple. iOS 11 is just really bad software. Not the calculator though, this problem is older and has been there in iOS 10 aswell.