I went to the Apple store today to test all the iPhone14 pro and iPhone14 pro max demonstration phones, all the phones are fine without any lag issues. . .
I don't know if it has something to do with the network connection, but when the connection (WIFI or Cellular wireless network) is weak, the stuttering problem is significantly exacerbated, and close the opening applications can help.
The reason is probably that demonstration phones at the Apple store don't have the same amount of apps installed and don't get used so intensively as someone's actual phones. I use my phone all day with a myriad of different apps. And that's when the problem starts happening. At the store people will only look at the usual couple of apps, and as far as I know, the iPhones are always reset overnight. So the problem will never appear.