I think we need to define "lag". I think we need to define "stutter". These words are being tossed around too loosely. For instance, my iPhone X lags a bunch. Tap the screen, swipe up to access my home screen, swipe through the home screens and its "lag" or "stutter" central. It doesn't happen every single time, but it happens a few times a day. It seems as though Apple has been apart of this "lag" and "stutter" fest for a few years now.
They had the crown ios 6 and below for being very fluent and stable. Once they started adding "features", that's when the hiccups started to show its ugly head. Definite crashes upon launching apps. OS would start to freeze here and there.