I don't mean to be so nit picky with the S8+, I would like to get one possibly, but maybe I am over sensitive to the potential lag, coming from my two recent phones being the Nexus 6P and now iPhone 7 Plus, that have really no lag whatsoever to speak of.
Maybe it's the SD835 vs Exynos version with lag and no lag ?
Honestly if you go in looking for lag, you will get probably find it even if it's very small amounts, and by the fact your looking for it, when you encounter such it would magnify the smallest amount of lag to be a bigger perceived issue than it is.
I think you have to approach it "well the device may have the occasional hiccup or stutter, but I will accept that for the extra features and hardware design / screen etc."
Otherwise your just setting yourself up for a frustrating unenjoyable experience.