That's a bunch of crap. Everyone's iPhone 6 will work exactly the same on the same OS on a fresh install. Same goes for everyone with a 5 or 5s.
Whether they see the lag or not is up to their perception and how sensitive they are to the issue.
I cannot test an iPhone 5 or 5s because I do not own either of them. So people who say it's perfect on their 5 or 5s could very well be telling the truth, but what I do know is iOS 8.4.1 was better on my iPhone 6 than iOS 9 and I've tried installing multiple times... some fresh, some upgrades and even tried resetting all settings (even tried 9.1 beta 1 and found it to be the same). It lagged on the beta and it lagged on the GM... and guess what, it lags on the final release as well.
It's not my particular configuration, it's the software. Stop trying to blame the hardware for the unoptimized mess that Apple put out.
Like I said before... just when I began to think 9.0 was perfectly smooth, I downgraded to 8.4.1 only to be blown away by how fluid ALL animations are and by how quickly it responds to any of my inputs.
Actually, you may find this hard to believe, but my iPhone 6+ on iOS 9 is experiencing terrible lag in the app switches. I might post a video at some point to show you, the animation stutters so terribly when scrolling between apps that it literally looks like a DVD that's skipping frames or a video game being run on a computer that doesn't have the specs to run it. It's so noticeable that when some of my friends saw it they immediately said it looks really weird.
Meanwhile, I've looked at 3 other iPhone my friends own. None of them had lag at all in the app switcher. I compared them side by side. Mine was stuttery, laggy and not as rsponsive, and theirs shows a smooth, clear animation going back and forth.
Meanwhile, my partner's 6+ has an identical issue on his app switcher. Same exact lag. But so far that's the only one I've seen that has the same issue as mine. I even went to the Apple Store today, where they had a 6+ I could look at with iOS 9 and it exhibited perfectly smooth animation in the app switcher.
Now the crazy part: I DFU restored my phone yesterday. Started it as totally new, no backup. The moment I got to the home screen and double clicked the home button I could tell the lag was STILL THERE. I let it go for 4 hours, loaded a few apps on it, set some settings. Restarted the phone. Left it alone for another hour. But the issue was still there. Even starting the phone as brand new on a DFU restore of iOS 9, my phone has a laggy app switcher whereas other people's iPhones (also 6+) don't.
Meanwhile my iPad Air 2 was flawless from the moment I updated. Go figure. I don't understand this issue at all and I hope they somehow fix it in a future update because my app switcher looks gross and is just not as responsive as it should be.