I can't speak about the iPhone 6 but the 6 plus suffers lag in different areas. Not only lags, but wonky stuff happens with it as well. I have to say iOS7 and 8 are the most buggiest softwares apple has ever put out. iOS7 was worst with frequent crashes all over the place. iOS8 has occasional crashes but it definitely suffers from lag and bugs.
Some body in these forums stated something that I totally agree with. I don't remember word for word...but he said "With android coming out with lots of features customers like, iOS is playing catch up and by them adding certain features, their disturbing the optimization that they are known for"
Clearly, 2gigs of ram should of been put in this phone. However, apple knows they will push units no matter what. So no rush to put those 2gigs in. They will most likely bump that up in the 6S and that to will push units lol. I ordered a Nexus 6 to test out L. Lets see how that goes