You know what's bizarre?
I've always felt the iOS keyboard was best during the iPhone 3GS days. I don't know what it was. It was just more reliable and more responsive.
I hated the keyboard during the 4S. Something about it got slower, less responsive, less reliable, and auto correct went to hell. I always coughed it up to my imagination (that the 3GS was better at typing) but it could be that expectations are now higher with competition offering better typing experiences (like Swiftkey)?
Haven't typed extensively on an iPh 5 yet, but I've always thought the iOS keyboard was pretty bad since iOS 5, maybe earlier.
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And for what it's worth, the keyboard is pretty bad on my iPad 3rd Gen too. Both on iOS 5 and 6. Constantly misses letters.