Nope, not like crappy iOS 11. Miles better and stable than iOS 11. Updated it and performance is smoother with similar battery consumption, slightly better standby times, on all devices a tested.
I usually reset after major updates, but did not do it yet to test stability of it and it's good so far on all MIUI9 devices. Even split screen is working reasonably good, which even iPhone X does not have yet.
MIUI9 unimportant notificatins folder is best. All unwanted notifications go to that folder without interruption and you can get back on them later on when free. Important notifications work as usual. Much needed feature on all phones.