That is what I thought, it needs further optimization to smooth out the issues. The iPad has 1GB ram as well right, with a retina display and no lag as I believe ?My guess would be that it's software. What needs to be remembered is that the resolution of the 6+ is still less than that of the iPad. The hardware is certainly more than capable of driving this display.
Not sure if lag is the correct term but there are some animation stutters occasionally when you do an action (the first time only).what lag?