Well, whatever the reason, the lagging is unacceptable on a 2014 flagship phone. I'm on the fence on returning mine (Verizon). It has lagged since it was taken out of the box.
This isn't 2011. I'm not OK with constantly clearing out RAM, restarts, or searching forums for solutions anymore.
To be clear, when I say "lag", I don't mean navigation or opening stuff is slow. I mean that when you select something there's a slight delay before it does anything, in many areas like Chrome and Facebook scrolling is choppy. The camera is often agonizingly slow to start up and capture, and the shortcut to it on the lockscreen is not very responsive at all. The answer/ignore buttons on the phone are not responsive either. It's often "I think I swiped it, did I swipe it, hmm, oh ok there it goes". There's more, but those are the ones that bug me the most. I think much of it is just a lot of sloppy or perhaps lazy programming in Touchwiz.