Should you not get your phone checked? A lot of people on this thread claim to have no problems with their 6+ (I have a 6 though)
Intermittent problems are very hard to demonstrate.
Also, seeing how 8.1.2 brought huge improvments I'm kinda hopeful for more. Nothing here is a dealbreaker, just disappointing. My phone isn't a lagmonster, it just has occasional WTF moments.