OneUI... TouchWiz is a very heavy skin compared to oneui.Ive used both too(from s2-s8) and used a grand prime for a while.TouchWiz never felt the same as snappy as oneui. What I like about samsung is they pack as much as features they can even we don...
Never experienced that before. I think its best if you can take them to the retailer you bought them from and claim the warrenty.Because im not 100% sure its a software related issue so it may be something hardware related.