My S6 is seriously just as fast, smooth and lag free as the first day I bought it, 10 months ago. I actually prefer the extra functionalities Touchwiz offers compared to stock Android. In my opinion, stock Android is a bit overrated. For example, I like how Samsung offers true multitasking (split screen). On the Nexus 6P you have a 5.7 inch screen and you can't even use two apps at the same time. What's the use of that big screen then? I use split screen all the time, for example just a couple minutes ago to watch the Note 7 event while browsing the MacRumors forum (see screenshot below). Also getting the latest Android update later isn't that big of a deal either. The phone will not suddenly become unusable or full of viruses when using an older Android version for a couple of months longer.
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Multi screen actually works much BETTER on the 6p if you are running Nougat, BTW. Multi screen on my Note 5 is neutered because the vast majority of apps don't work with it. Even Samsung branded apps many times don't work with multi screen, which is supremely idiotic IMO. With Nougat pretty much anything works in multi window on the 6p, it's very impressive. Finally I can have apps like Waze in a multi window.
My Note 5 lags as well, but it's intermittent. Sometimes it's very smooth, other times it does pause for a second when transitioning, no rhyme or reason. I've never seen a lag on my 6p, but I've run it mostly with nougat so maybe it's that.