iPhone 6 Plus Teardown
ifixit.com
ifixit.com
2011 iPhone 4s iOS 5
2012 iPhone 5 iOS 6
2013 iPhone 5s iOS 7
2014 iPhone 6 iOS 8
which suggests that it'll be iOS 11 that the iPhone 6 starts struggling with. That said, I expect you'll have more trouble with the 6 Plus because it's trying to push more pixels around with the same hardware.Which is exactly what most people used their (not at all locked down, similarly powerful) laptops for before smartphones existed.
http://www.laptopmag.com/android-sticks
With a little hackery you should be able to do pretty much everything within the storage / processing power budget.