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Normally ArsTechnica do a decent benchmark on this after each release, however this year, they've decided not to:
You might be concerned about the performance of new software on your old device (though it's been years since a new version of iOS made older devices feel intolerably slow, which is one reason we aren't testing iOS 15 on old devices like we usually do).
Ars article

Might sacrifice my old 6S to find out later.
 
Installed iOS 15 on my iPhone 6s. Seemed laggy. Restored iOS 14 IPSW while it was still signed. I'm good with 14.
 
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It seems to be running ok on my 6s. The colors look kind of off now though, especially in Safari. They are kind of washed out and some graphics look grainy. I might roll back to 14.8.

Since they are now officially still supporting iOS 14, does this mean they will continue signing 14.8 for the duration of the support period? Or do I have only a couple days like before?
 
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Just adding my experience.
As I’m getting a new iPhone in the next couple weeks, I decided to take the plunge on my old trusty 6S.
I did an in place upgrade on top of 14.8 (last clean installation of iOS was in February 2020) and so far everything seems to run just as smooth as it did on iOS 14. Once or twice I saw it stutter, but I believe it’s because the phone is still running background tasks related to the update. Overall I don’t feel it to be slower than iOS 14.
In terms of battery life, it seems to be about the same (if not slightly improved once all background tasks finish running?), and I’m getting around 2.5~3.5 hours of screen on time on a charge (it has a brand new battery replaced less than a month ago).
Overall I think it’s safe to upgrade. But again, the 6S barely gets any of the new features, so it’s not like you lose much if you stay on iOS 14, especially now Apple committed to support it with security updates for a while. But as usual, YMMV

mtrm
 
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It seems to be running ok on my 6s. The colors look kind of off now though, especially in Safari. They are kind of washed out and some graphics look grainy. I might roll back to 14.8.

Since they are now officially still supporting iOS 14, does this mean they will continue signing 14.8 for the duration of the support period? Or do I have only a couple days like before?
No sure thing about what Apple will do (keep iOS14 still signed). Make the move back to 14 if you have reservations. When Apple no longer signs 14, that door will close.
 
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My experience on my XR with 15:

Very laggy and seems to run out of memory for apps way more than 14. iOS 15 Safari is particularly slow when switching between tabs.

(Also the UX is terrible - but that’s simplify a bad experience on whatever phone you have, IMHO).

Downgraded back to 14.8 after 48 hours and my phone is again fast and snappy.

And although this is off topic, I reinstalled Big Sur to get rid of Safari 15 - simply because of the appalling UX.

Maybe 15 is ok on the 13 - and maybe 12 - but until Apple does seem serious optimisation work, I’m not going near it.

I thought it wasn’t possible for Apple to get any worse than the debacles of iOS 11 and 13 but this is a new low.
 
I installed iOS 15 on my SE 2016. Had to offload just about every app to clear enough space to do the download and install. Probably the slowest install I've done. I was worried because I wasn't able to do a full backup beforehand but it eventually completed.
Everything seems much as it was before. I'm not noticing any stutter but it's running quite hot at the moment and the battery is running down fairly quickly - but the battery isn't in the best of health.
I updated my Air2 yesterday. Also no noticeable glitches.
 
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Hi, I tried ios15 on my 6S, but it was too laggy. I made a backup complete with iTunes and I downgrade software to 14.8 installing .ipsw file with iTunes.
Now i have two big problems: i have recent Photos that they are reversed: recent are up and older are under.
But the the biggest problem is that the phone fails to backup to iTunes and fails to upgrade to return to ios 15 with iTunes.

I tried also to restore iTunes backup, but it says no existing backups.
Can you help me please? Thank you
 
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I think I’m going to roll back. I’ve been using iOS 15 on my 6s for a week and my battery life isn’t great and the phone is very laggy. I’m getting random freezes and crashes also. I’m sure some of this stuff will be fixed, but in the meantime I’m ok with 14.8. I originally did an upgrade install but then tried a DFU install. Didn’t notice much difference. It was worth a shot.
 
I run it on my 6s without any problems. No freezes or crashes and no lagging.
My battery life ain't great but this can be just my aging battery that hadn't been better before.
What I really like is the close all tabs feature although I would even prefer to not need to confirm the closing each time. Not sure how I like the lowered search bar.
 
I switched back to 14.8. My 6s was getting a lot of UI lag and delays/glitches when typing. A DFU restore didn't fix it. 14.8 works fine for now.
 
I’ve been running iOS 15 on my 2016 SE since dev beta 1. Buggy in the early days but no problems now. Just as zippy as 14. Feels like battery life is better but I have no data to back that up.
 
All I can tell you is that iOS 15 on my 12 Mini has waaaaay worse RAM management than 14.8 did on my 6S. Apps reloading all the time on the newer and more powerful phone when they didn't hardly at all on the old sucker.
 
All I can tell you is that iOS 15 on my 12 Mini has waaaaay worse RAM management than 14.8 did on my 6S. Apps reloading all the time on the newer and more powerful phone when they didn't hardly at all on the old sucker.
Are you using the camera often? I find that it's one of the more RAM-intensive things..
 
It worked as well for me as iOS 14... Then I went and bought a 13 Pro and had my world blown open.
 
I have iOS 12.4.1 on my 6S plus and haven't upgraded (from personal experience from the iPhone 3 and 4S, I have learned you can only upgraded about 3 full versions max before slow, laggy weirdness). The issue I have having now is every single week or so, an app no longer works and needs at least iOS 13 (i.e. Amazon, Best Buy, Home Depot, Lowes, Walmart, etc., etc.) Is there ANY way to upgrade to iOS 13 or maybe 14 and not the latest iOS 15?
 
Still on iPhone 6s? After more than 6 years it may be time to buy any iPhone 13.
 
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