Hi @DottyD
May I suggest to try these generic troubleshooting guides.
> Safe Mode > https://www.samsung.com/uk/support/mobile-devices/how-to-start-my-galaxy-device-in-safe-mode/ turns off downloaded apps and themes etc which may highlight an app or theme that's causing conflict.
> 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.
Feedback and any relevant error reports can be sent via the Samsung Members App under the GetHelp section.
If these do not help then as a very last resort try backing up and performing a factory reset.
A Samsung Experience Store or Samsung Service Centre can check out your phone.
I wish you all the best. 
