Name one app that Samsung does better than Google ? And no S-Pen stuff, because obviously stock Android doesn't do that.
To be honest I despise Touchwiz, Lagwiz, or Grace UI, whatever they call it today, it's still a super bloated OS. Look in Developer Options, under "Running Apps" holy cow Batman there's like ten million apps running in the background on a Galaxy phone. Do they same on a Pixel, and it may be a dozen or so. On my Nexus 6P I have 9 services running in the background. Point being, when there's like 20+ services running in the background that contributes to micro stutter and slight lag.
I owned the S8+ for a week, and FACT it still lags and stutters, sure it's minor and only does so very little for a split second here and there, but it's there. Sure it's way better than horrid S4 days, but it's still there. My Nexus 6P, ZERO lag, warp speed smooth.
Read the S8 forums, now that the been out for almost siz months, there's plenty of posts about lag getting worse as the months pass by with the S8. Not making that up