I can admit when I made a mistake. As for a placebo effect, I have access to 2 phones: a 6 and 6+. Back when I was running iOS 8.1.3 I ran the same apps from the same iCloud back up (which support told me was the preferred method) to test both phones. The 6 smoked my 6+. I bet people here, like you, will say I was imagining this. Safari didn't behave the same. Ran diagnostics on the 6+. Nothing was wrong and I was told to wait for an updated version of iOS or updated 3rd party apps.
As I stated earlier, I don't know if the improvements are caused by Apple fixing iOS or updated 3rd party apps. Given Apple's track record of late, I guessed that it was iOS. Did you run every beta? I did and I saw Safari improve right after updating to the next beta. And speaking of release notes, I've seen improvements that weren't mentioned in any change logs. I saw Apple improve and then remove the improvements to app switching that finally made it feel as fast. Don't believe me? Search for my post history. I complained about this months ago. Oh, but I bet you're going to say I was just imagining that performance improvement.