All these threads? There are a bunch of whiners and millions, literally, of satisfied users...
From the tests I've made (playing with both in an Apple Store for more than forty minutes doing almost every thing but phone calling) there is absolutely nothing laggy in the new iPhone, compared to my iPhone 5S.
----------
Blame the keyboard you installed, not Apple.
I'm using Swype on both the iPad and the iPhone and I didn't have a single failure.
Well I've seen 3 or 4 threads that were quite active about it, I just assumed either some people might not care enough to notice. I just played with a display 6+ and I thought I noticed some but I didn't get to use it long I need to go back and use it again.