The iPhone XS and XS Max will experience slight animation stutters after about ten seconds of inactivity. This is usually most visible when swiping to go home after reading static content in the foreground app for more than ten seconds without touching the screen. We’ve been tracking this issue for a few months now and the problem continues to be present in the iOS 12.2 betas.
Seemingly most, if not all, iPhone XS and iPhone XS Max units experience this problem. There have been pages of Apple forum threads since the XS launched in September 2018 complaining about this issue. Someone has even made a dedicated XS Stutter Facebook page to air their frustrations.