iOS 11 is the first time that I am seeing "delayed typing/keyboard lag" on an iPhone.
This brings back the horror memories from Xperia days of mine.
Seriously, I don't necessarily type that fast but letters take 1sec to show up and trust me it doesn't sound that bad, but the lag is extremely noticeable, something never happened to me on iOS.
Edit - only happens on 6 and not Air 2, and I know the device is 3 years old but I'm not sure how task heavy is "keyboard" enough to lag when texting someone.
Edit 2 - Just updated it, and now this one feels extremely stable and no keyboard lag YES! I love Apple again lol.