I've been on Android since the Nexus-One days. I was a proud owner of the N1 when it was new, and stood in line at the T-Mobile store the day the Nexus-S first came out ( second Nexus phone ). I have owned pretty much every Nexus phone, and also a lot of Samsung Galaxy phones over the years too. The S line I had the S2, S4, and S8+, and the Note's I had the Note II, and Note 5. Out of all those phones, for whatever year it was the Nexus of that year compared to the Galaxy or Note of the same year, was always smoother, less laggy, and less hiccups.
The S4 was a **** phone, and the Nexus 5 compared to it, was night and day smoother.
I bought the new T-Mobile Galaxy S8+ this Spring, and returned it a week later, it did indeed have real lag, and slight micro stutters FACT, not made up. It was not as smooth and lag free as the Pixel XL.
Another example, my Wife and I both bought brand new Note 5's the week they came out, and brand new they were amazing phones, fast, and smooth, and Touchwiz seemed much improved. And then I got the Nexus itch again, and picked up the Nexus 6P on a whim, and will admit it was even smoother, than the Note 5. While my Wife kept her Note 5, I noticed like six months in, it felt like an old Windows 95 PC, totally bloated and laggy, and stuttering. So I performed a full wipe, and complete system fresh install on her Note 5 and that helped a ton to make the phone smooth again. Fast forward three months later, and the lag was back again. Compare that to my nexus 6P, and six months in, it felt like the day it was brand new, still smooth, still very little to no lag.
I really love the hardware and cool designs Samsung has been doing lately. But fool me once, and not again. I just can't trust a Samsung Galaxy phone will still buttery smooth and lag free 6+ months on. I do not understand how a Pixel phone can, but a Galaxy cannot ?
Oh and the Galaxy S7 Edge, wow that phone performed liked the S4, and looked like something out of Gingerbread days.
I did not actually own the S7 Edge, but in my office, I was tasked with settings up everyone's brand new S7 Edge's they got earlier this year. Out of 8 brand new Sprint S7 Edge's all of them were lag city BIG TIME. I was really shocked just how much micro stutter and slight hiccups they all had. Granted this is during the setup process, so tons of background downloads and installations were all going on at the same time/ on these phones, but even after I had them all setup, totally updated, and the apps installed they liked, yes it smoothed out a lot better, but that slight hiccup was still there. Pull the notification shade down, and sometimes you'd get a half second delay. Or go the Play Store, and maybe a 1 second pause before it opens up. To me that's totally unacceptable, a modern smartphone should perform actions faster than a blink of an eye, and never lag, never pause for half a second, never hiccup, it should be smooooth ALWAYS. And to me, the only phones that are that smooth are rooted and ROM'd Nexus / Pixel phones.