Lag and stuttering are starting to take a toll on my 6 Plus and with the same setup I had on my 5.
I had an iPhone 5, and also have an iPad Mini with Retina. The A6 chip had more available RAM than the A7 and A8 chips because they are 64-bit, and 64-bit apps use 20-30% more RAM for the same tasks. I doubt you'll see much improved performance on the iPhone 6, as my iPad Mini had issues with browser tabs reloading and some occasional lagging. Here's the deal: The iPhone 6 Plus is faster than the iPad Mini Retina and iPad Air, and they are pushing a resolution that is much higher (even when you take into account the scaled version on the Plus). The iPad has occasional lag, but it's not as bad as you're saying. I can confirm this, as I have an iPhone 6 Plus with occasional lag, but not as bad as you're saying. So either you have a defective device, and/or Apple needs to put out a software update. Did you run iOS 8 on your 5 before getting the 6? I did and had some occasional lag, especially with messages and sometimes with the keyboard. I'm leaning towards iOS 8 being buggy right now. We are talking about a point zero release here. Happens nearly every year. The other thing is sometimes when you transfer backups the settings file gets corrupted. I've had several genius bar guys tell me that what fixes most of people's problems with new iOS upgrades is to reset all settings. NOT reset content and settings, just settings. It's stupid and Apple should fix it, but I would try doing that before returning it.