I have 8.1.1 now on my 4S .... and the performance is getting better (Although not as smooth as 7.1.2 was)
The biggest improvement for me in 8.1.1 is the camera app. Especially when switching to the camera roll view via the little thumbnail image in the bottom left corner of the cam app. With 8.0 & 8.1 it would take an age to view the photo just taken using this button. Now it's much much quicker. Dare I say back to normal.
This improvement alone make the update worth while for me.
Everything feels snappier for me now after the 8.1.1 update on my older test devices. I think there must be some background tasks performed after upgrading because initially it wasn't impressive. But now the keyboard lag is significantly reduced, and Safari is usable, moving around the OS feels far smoother than before. This is probably as good as it's going to get for iPhone 4s/iPad 2 users.