iOS 9 public beta 3 is also stuttering on my standard iPhone 6. Especially the multitask menu seems like it is designed for 2GB RAM or more. Personally I don't care about reloading tabs in Safari, as new updates to the webpages I visit is usually a plus, but if you are on a slow connection it will also make your phone seem static.
The bigger screen on the iPhone 6 Plus also makes the lag, freeze and hiccups more visual. If you look closely on your standard iPhone 6, it will struggle in many of the same scenarios as the iPhone 6 Plus, but the smaller screen simply "hides" it within the form factor. Though the lack of fluid performance is evident, I still like to use the bigger screen and may say I prefer to actually use my 6 Plus over the standard 6.
I really love making playlists and using Apple Music on the Plus, but I am afraid to keep it. The announcement is only two weeks away, and the wait to actually get a phone may be around late October (Norway). The iPhone 6S Plus will also cost me 200 dollars more, because of a great deal I got from the shop I use (around $549 sim-free).
If I am allowed to compare apples to pears, I may say that the new retina Macbook also have performance issues like lag, freeze and hiccups, but I no longer care about it. It's sufficient for the tasks I use it for. I really love the small footprint. May be tempted to keep the 6 Plus and wait for iPhone 7 and iOS 10. As an Apple fan, I am also interested in showing off that I have the latest and greatest, and the little 6S print on the lower back of the phone may not do it in a pumped up fashionista regime.