That's almost too funny. The conclusion on the video was sad as sometimes 9.1 was faster (ex. app switcher closing apps) and sometimes 8.4.1 was. It certainly wasn't slow by any means, especially when you add in the extra functionality it offers (like extensions, low power mode).
Besides, that's one device. 9.1 beta most certainly works better on my 6 Plus. 8.4.1 gave me a crashing camera app and RAM restrictions where the apps wouldn't remember their state. THOSE were major issues.
9.0 GM has stuttering issues but at least it solved my two major issues. I'd rather have that than crashing apps. 9.1 beta seems to have rectified most of the stuttering for me. Sadly I suspect that's as good as it'll get. The 6S Plus will probably fully fix all the speed and RAM issues. One of these days I'll get on the S cycle.