This seems really odd to me. I just got an insurance replacement and I had been playing around with iOS 11 which I didn't want to continue with so I ended up with no viable backup to use. I set it up as a new phone and added my old appleID to the phone the re-downloaded all the apps I had before that I actually use.
I had a 6 up until about 11 months ago when a friend was selling his cheaply and I wanted to see if the plus size model would be too big so I traded from a 6 to a 6+. The 6+ immediately felt faster but since I did a clean install 2 months ago the whole phone has been quick.
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Phonzoxd - I just tested to see how fast it was to open the camera and start taking a video and from having the app completely shut down. It took about 0.5-1s to open the app and show the live viewfinder. It then took literally 0s to start filming a video. It sounds like people who are having issues with their 6 have software or appleID/account problems.
Is there anything else in particular that you find slow that I can try on mine?