Issues and inconsistencies and the like do not on their own make for a conspiracy (while they can make for a conspiracy theory, or two, or three, just like pretty much anything can).
Performance declined from ios 6 to iOS 7 and from iOS 7 to iOS 8.Performance heavily declined from iOS 8 to iOS 9.Now performance DIDNT decline from iOS 9 to iOS 10
UI/UX redesign from 6 to 7, under the hood redesign opening up much more from 7 to 8, something that didn't work out as planned from 8 to 9, and things improved and more from 9 to 10. Though apparently "some people" see iOS 8 as a great version and compare things against it, while many others don't really see iOS 9 really any worse off than iOS 8, so it seems like even what you mentioned doesn't quite hold, and even for those that it does, there are certainly plenty of realistic possibilities outside of a "conspiracy".
My experience: iPhone 5s, 32gb, public beta 3 for 4 days. My old phone my dad has now. I backed it up and upgraded to iOS 10 and let my dad use it for a day just to get his feedback (and annoyances). It was sluggish, noticeably. But he was using the camera and using iCloud and Siri. Maybe it's just re-indexing etc? But the next day he said that was gone. I used it that day for 6 hours as a second camera, and it was just fine. Compared to the 6s Plus I have, iOS 10 feels a bit less responsible. But I have the same with iOS 9. It's certainly not the case that 10 is slower than 9, but most certainly not faster. It however does make me 'feel' that 10 is going to be the last OS for that phone. The 64bit support on the 5s is quite impressive. We both didn't think about it anymore on days 3 and 4 that it was 10. It just felt like a normal phone again with an update to iOS9. Not a moment I went 'ugh, this 10 update.. sigh'. Just for stability reasons and to not confuse my aging father with new things in apps I restored it to 9.3.4 again.
