I've owned an S8+ since the day it came how, and have ended up having it replaced once.
While it was pretty awesome on launch day, it has since deteriorated as terribly as most Samsung phones do after 6 months or so - the animations seemingly are just not even designed to be as smooth or fluid as stock android - frames constantly drop, lags happen regularly, and most obnoxiously for me, the more use the phone gets, the longer it'll take to open apps.
Whereas the Pixel will open an app as soon as you tap it, there will be a noticeable delay on my S8+ between when I tap it, and when it opens. This can vary depending on the app, and if it's still in memory and whatnot, but it's pretty consistent and pronounced.
I've also experienced the lag and input delay with the keyboard randomly and seemingly without any obvious reason. Sometimes it's after heavy use, sometimes it's after just a few minutes of use.
In some cases and at certain times, I've actually found budget phones like the Moto G5 Plus to run faster and more reliably than the S8+, which is pretty ridiculous when you think about the difference in price points.
With Samsungs, it just seems like absolutely everything is great on the hardware side, but the software is just an utter mess - it's like nothing is optimized, no thought is giving to long-term sustainability, and the minute the phone is out of the box, the software experience starts to deteriorate rapidly. I honestly started considering switching to an iPhone just because I was THAT annoyed by the lag before I decided to just explore other options among android phones. I honestly don't quite understand how so many phone reviewers and users can look past this many software shortcomings.