Have you tried a hard reset of your iPhone 6/6S by holding down both the Home and sleep/wake buttons until your iPhone goes to black and restarts with Apple logo, then release the buttons?
How much free data storage space is left on your iPhone?
iDevices need to always maintain, at the very least, between 2-3 GBs, OR GREATER, of free data storage space.
In Settings app, under General settings, Reset panel, at the right bottom of the list, Reset All Settings.
In Settings App, Battery, check to see if any apps/processes are eating up your battery.
In Settings App, General,Reset, Reset Network Settings, try resetting your network settings.
In Settings app, General settings panel, turn OFF Background App Refresh for any apps you feel do not need constant data updating/Internet updating in the background when not using the an app OR turn OFF Background App Refresh entirely/globally for the entire device.
Do the same in the Settings app under Notifications.Turn off notifications for apps you do not think you need notifications for and/or edit the different ways you receive notifications to help with performance by not always using all the notification methods together.
In Settings app under Mail, you may want to set a longer fetch time or set your iOS Mail app to only manually fetch your mail ONLY when you launch the iOS Mail app.
In Settings app (NOT from the iOS Control Panel) turn OFF Bluetooth when not using any Bluetooth devices.
Also, in general, if you want a faster IDevice, on all of my iDevices, I turn off most of the iOS motion graphics eye candy, by simply turning ON Reduce Motion in Settings app, General, Accessibilty settings.
Make sure you aren't running 30 -50 OR EVEN MORE background apps in the iOS 11 Control Panel/App Switcher.
If you are, you need to quit the bulk of these background running apps by tap and hold a finger on an app window in the switcher and slide your finger upwards to quit an app. You should be able to use more than one finger to quit more than one app window simultaneously.
Also, make sure you don’t have 20-50 OR A WHOLE LOT MORE of active website tabs running in the web browser.
If you do, greatly reduce the amount of active website tabs your web browser.
Some minority of user have been turning ON and leaving ON iCloud backups continuously backing up on a constant and daily basis.
If you are one of these users, in Settings App, tap your Apple ID, then tap iCloud, then find iCloud backup and turn off iCloud backup and only turn this on to do daily backups either when you are not using your iPhone for a prolonged period of time (like when you are sleeping), then turn iCloud backups OFF when actually using your iPhone.
OR
Only perform your backups on a lesser time schedule.( like twice a week, once a week, once every two weeks, once a month, etc.).
Any large amount of background running apps or active processes that have to constantly go out to the Web to fetch data ALL THE TIME is going to slow down ANY iDevice and can, also, negatively impact battery life.
In addition to all of the possible fixes and solutions presented, another iPhone 6S user, in another single posting in the iPhone support communities, suggested to go into Settings, General, Accessibility, Increase Contrast and turn ON Reduce Transparency.
Your iPhone 6/6S won’t look as pretty, but that user reported that his iPhone 6/6S speed improved considerably.
If none of this, so far, has helped your iPhone 6,/6S,
What about doing a complete backup to iCloud or iTunes on a computer, then do a complete erase and reset as new in the Settings app under General, Reset then restore your backup from iCloud or iTunes?
If still no joy after the erase and restore, hammer Apple with constant feedback about the iPhone 6/6S issues here.
iPhone Feedback
http://www.apple.com/feedback/iphone.html
This is THE ONLY avenue to get Apple’s DIRECT attention to any and all iPhone 6 issues!
IF no one continues to post regular feedback about iPhone 6/6S issues, Apple will NEVER know there is an issue and will never correct it!
Cognizant Apple employees read ALL feedback generated from all of the various feedback pages and transfers the data to the appropriate Apple personnel , but NO Apple employees will ever respond with any type of direct, individual replies from the feedback you post.
Good Luck to You!
