My 2 cents. That doesn't show to me a defective phone. That just shows me some app is using a lot of RAM and CPU to the point where other functions are affected. A smartphone, while a phone, is still technically a computer. You can overload your computer easily by going from 0 to 100 MPH. Its as easy as selecting all desktop icons and pressing enter.
Anyways, send your phone in and get a "new" one. It will very likely be a reconditioned phone but there isn't a difference if its Apple that completed the repairs.