In the iPhone X video that started with Johny Ive talking about the design? It definitely looked like there were dropped frames/stutters with swiping to unlock and going through the multitasking sections. Hopefully this isn't the case.
Yep, I noticed them too, especially when closing apps and going back to home screen
new iOS = new bugs
it'll never be perfect
I do always wonder if they are using a final production device for demos, let alone a final and polished version of software. Logic would tell us they should, but you never know, especially when the device is brand new with new tech.
OP.. Apple was known for smoothness. They were known for a lot of things that are no longer the case. iOS stutters now, that's how it is when you design software to work across 30 devices instead of just one.
I certainly wouldn't judge it off of that. My iPhone 7 Plus runs smooth as silk. The X is much more powerful. The final version of OS 11 should not have any trouble on it.