your device is just old, it was optimized for iOS very well actually. Your phone is just old so it won’t run well, it will only run well if you have iOS 13 or above and probably ios12, based on your device name it is probably running ios11
i am using ios 12. Although it works on iphone 6 (no longer updates to ios 13 including iphone 5s). The iphone 6 is slightly faster than iphone 5s even though they have the same ram. So its most likely a code issue
