All of you here should be constantly posting multiple feedback to Apple’s iPhone feedback pages and voice all of your iPhone 6/6S issues and problems there.
I do not know why we have a growing minority of IPhone 6/6S users having serious performance issues under iOS 11.
This is not a world-wide, user-wide IPhone 6/6S problem with iOS 11.
Apple designers and engineers do not read nor participate in these Apple technical support communities.
These are user to user Apple product support communities.
We are all users here, just like you!
Apple supplies these forums for users to work out their product issues with other users, but these community forums ARE monitored/moderated by Apple Staff assigned to these community forums, but I do not think any info is ever passed on to who/mever the cognizant parties are who would need to know if there are any mounting issues with a product or software.
The only way to get Apple to listen to you is to use their feedback portion of their website.
iPhone Feedback
http://www.apple.com/feedback/iphone.html
Cognizant Apple employees read ALL feedback generated from all of the various feedback pages and transfers the data to the proper, responsible Apple teams and personnel , but NO Apple employees will ever respond with any type of direct, individual replies from the feedback you post.
Also, have all or many of you loaded up your iPhone 6/6S with lots of apps and photos?
Do any of you run your iPhones with scads and scads of Background running apps?
If you have more than 20-30+ apps running in the Background, try quitting apps to reduce the amount of Background running apps.
In Settings app, General has anyone tried turning OFF Background App Refresh?
Scads and scads of active website tabs in the web browser?
Reduce the amount of active web site browser tabs.
How much free data storage space is left on all your iPhones?
At a minimum, everyone should be maintaining a minimum of 3-4 GBs of free data storage space.
IPhone 6 models only have 1 GB of RAM.
IPhone 6S models have 2 GBs of RAM.
With the IPhone 6, users do not have a lot of RAM resources to have lots of stuff running in the Background and/or having scads of tabs open and active in a web browser can cause issues, too!
Also, found these videos on iPhone 6/6S showing the iPhone 6/6S running iOS 11.2 and iOS 11.2.1.
These two iPhone 6 look to be running pretty well on iOS 11.2 and 11.2.1
iPhone 6
https://www.youtube.com/watch?v=5IV24uzzmxY&t=425s
IPhone 6S
https://www.youtube.com/watch?v=5IV24uzzmxY&t=425s
Good Luck to All of You!