If you are running iOS 11 you may need to force restart your iPhone for different reasons: getting slow is one reason and you can a fresh boot with a force restart.
For different iPhone models we have different Force Restart Methods. The main reason is because some iPhones have solid state home buttons which means the click is mimicked by the software. The other reason is the new iOS 11 features update which as a new shortcut for Emergency calls.
iOS 11 Force Restart / Force Reboot iPhone
1. iPhone 6S or bellow (with clickable home button): Press the Home + Power button together until you see Apple logo.
2. iPhone 7 (with solid state home button): Press the Power button + Volume down together until you see Apple logo.
3. iPhone 8, iPhone 8 Plus and iPhone X: 1) click volume up; 2) click volume down and hold the Side Button until you see Apple logo.
Important: Apple renamed the power button or sleep / awake button to a “Side Button”. The method used on iPhone 7 can’t be used because Apple pressing Power button + Volume down together on iPhone 8 and iPhone 10 will make an Emergency call.