The mixed feedback make me suspect that the issue is with some widgets/apps installed on the device. Maybe trying to delete one for one can be a thing to investigate the issue
This. My XS Max is fine on 15.4, no stuttering etc. When people have serious issues with performance and others don't it's almost always one (or more) of a few things:
1) One or more apps that are misbehaving. Update all apps, check things like battery usage to see if there's an app appearing there that has no apparent reason to be using much batter.
2) Hardware. I'd especially look at free storage. I don't know this, but it wouldn't surprise me if running at something like 98% full compromised performance.
3) Something subtle wrong with the OS or other software. Do a clean install after backing up. REALLY clean, not a restore from backup. Yes, this is a pain in the ass.
COULD it be an iOS issue? Sure. But when it's not everyone on your device model, it's more likely to be something about your setup.