If you got your hands on the iOS 12 developer beta, and have been religiously updating it over the air until the notorious beta 7, you'd be (fairly) happy to hear that's the reason your phone slugged out. Even on the fast and furious iPhone X the iOS 12 beta 7 update causes unresponsive lock screen, apps that take seconds to start loading, and a laundry list of daily annoyances. That same beta was supposed to release group FaceTime chat features, but that's been postponed for a later date now.