I'm running in Quad HD resolution with the OOTB theme that came with it. Using the stock Samsung launcher. Apps that would eat up lots of memory include Facebook. I don't use the stock Samsung email client, instead opting for Nine Email, but I do use all the rest of Samsung's apps such as their messaging, calendar, contacts, etc. I have a Class 10 U3 Samsung Pro 128GB microSD card for photos, videos and downloaded songs from Google Music. I have a few games, but rarely play them, and I use Waze, but shut it down and swipe it away as soon as I'm done with it.
I have Package Disabler Pro, and have disabled T-Mobile TV, and T-Mo Name Service, along with Hancom Office (since I use MS Office). Earlier today I disabled Bixby and so far haven't had lag. But since the lag is very intermittent, I'd have to run it for at least another 24 hours to know for sure if that made a difference.
Since I disabled all things Bixby using Package Disabler Pro, the lag has not returned. As a side note for those folks suggesting that Facebook could be the culprit, FB's app is terrible in terms of memory management without question, but I've used it on many phones with it causing zero lag. I don't believe that it really causes lag overall. To name a few phones that I either have or had where FB didn't cause lag: OnePlus 3T, Huawei Mate 9 (both of these absolutely fly compared to other flagships), Pixel XL, LG V20. If FB didn't cause lag (it did use up battery life), in any of these, it's not causing lag in the S8+.
Anyway, for now, I'm a happy camper. This screen is unbelievably beautiful.