my iPhone 5s and iPhone 6+ both running the latest iOS (11.4) the performance is good, battery performance is same. Planned to get iOS 12 public beta. I didn't found the "hot issue" here. I'm using my phone for social media's, spotify, sometimes games and another productivity. Are you using the phone while charging? if it's no, try to borrow an original charger, if still, something wrong with your phone. Hope it helps!