Force restart iPhone 6s
Press and hold both the volume down button and the Sleep/Wake button at the same time. When the Apple logo appears, release both buttons. (DO NOT release Side Button when invited to Slide Power OFF).
If the above does not resolve the issue then please check your iPhone's available free storage space. Open Settings ➔ General ➔ iPhone Storage: Here, you can determine the total available free space. If you have less than 6GB available then you may experience poor performance issues such as the one you have mentioned in your post.
The other thing to note is that your display may be failing. If that is the case then have your iPhone examined at the Apple Store or at an Authorized Service Center for further diagnoses and resolution.
Axel F.