Be patient. I have iOS 11B5 and it is much improved. Battery life is close to 10.3.3 (I have archived screenshots to compare.). By close I mean 15 min difference over a day usage.
It still has lots of stupid bugs. Calculator is just broken. Misaligned text. Rotated screen when awakening. Things like that.
Speed is fast on B5 also. Compared to the sh&$show that 11.0.X was it is refreshing.
Random freezes, phone calls not having sound, and for me as a podcast/audiobook listener that side is awful, widget not working, having to restart the app or the whole phone, bluetooht connectivity being very unreliable. Usability gone much worse regarding audio.
In situations where with ios9 I could rely that I put my headphone on and push the button on the headphones the correct thing would start playing immediately this got worse with ios10, things started to not work always and with ios11 it's so bad that I can't bring myself to spend money on a new ios device. When my 6s dies I hope there will be something other then the duopoly available.
It is as bad as you report (I run both a 6 and a 7 as daily drivers). I'm running the latest iOS 11.1 beta on my 6 for it to be usable, highly recommend doing so; its superior to the non-beta 11.0.*
The situation is perhaps even worse on iPhone 6, which I have. It feels like Apple doesn't test iOS on older hardware. For example the camera app has rotation freezing problems [1] half the time I open it. It's not a rare occurance, it's a constant issue. It's not a case of slow hardware either, because there are still plenty of times when everything works smoothly.
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[1] When I rotate the phone, half the UI elements react to the rotation and update, but the other half of the UI remains at the previous mode. So I have a UI which is half landscape, half portrait. Depending on the mood of the iPhone, the UI will fix itself in either a second, 5 seconds, or not until I restart the app.
If you're using Apple CarPlay, don't upgrade (yet?). On iOS 11 it cannot even keep up playing music without glitches. Crashed twice on me when receiving a phone call. Never had those issues on iOS 10.3.
Rebooting helps. Feels like trying to use Windows on a cheap desktop to watch an HD movie in 2001.
Basically the iPhone 6S was perfect as a phone (meaning I can see myself using it for the next 5 years) until the update. Now some apps will freeze and it gets really annoying. I've seen my friends using it on an iphone 6 and you can't even text anymore.
My wife and I both have iPhone 6s. She upgraded and has regretted it since. Very slow wake up, unlocking, and app switching are just some issues. There's a lag with most actions. Random app crashes (even those updated), and battery life has dropped significantly.
Ios11 is a disaster, half the battery life of previous versions, safari video looping, and constant lock screen crashing. First time I’ve been unhappy with my phone.
It still has lots of stupid bugs. Calculator is just broken. Misaligned text. Rotated screen when awakening. Things like that.
Speed is fast on B5 also. Compared to the sh&$show that 11.0.X was it is refreshing.