Worth a read. It addresses the reality of the situation and ways to help:
http://bgr.com/2017/09/19/ios-11-features-iphone-how-to-speed-up/
"Apple has another big release that takes place today. That’s right, iOS 11 will be released to the public on Tuesday, and hundreds of millions of iOS devices around the world will instantly have access to it. Thankfully, iOS isn’t like Android where people have to wait months before they can actually install new updates.
iOS 11 is packed full of new features, enhancements, and behind-the-scenes tweaks that combine to dramatically improve the iPhone and iPad experience. Of course, iOS 11 also has some changes that people won’t appreciate, and a potential slowdown on your iPhone is likely at the top of the list. Don’t worry though, because in this post we’ll give you a few different ways to speed things back up..."
http://bgr.com/2017/09/19/ios-11-features-iphone-how-to-speed-up/.
"First things first: after you update your iPhone to iOS 11, don’t panic. Your phone is going to slow down and your battery life will take a hit for at least a day or two as your phone re-indexes. After that, you’ll see a marked improvement in speed and battery life… probably.
Where speed is concerned, some older iPhone models will never run as fast on iOS 11 as they did with iOS 10 installed. Efficiency improvements in upcoming iOS 11 updates could help things down the road, but right now you’re stuck. That said, there are a few things you can do to speed your phone back up if the slowdown starts to get to you..."