Clear Partition Section _ Clear Cache Files from the phone's System Partition Section > https://www.samsung.com/uk/support/mobile-devices/how-do-i-clear-the-cache-on-my-device/ You may need to insert your earphones or hook upto a powered On Laptop to access the Clear Partition Section. This may help with lag or stutters as this clears the phone of old/unused or broken cache files.There is also a Repair Apps option in the Menu System too.
Factory Reset as a last resort after backing up.
Let a Samsung Service Centre have hands on or take it back to who supplied it if not Samsung directly.
There maybe a cooling off period to have it swapped out.
If your past this stage then they'll look at a Repair as long as no water is in the phone or it shows signs of an accidental drop / bump.