Apple finally blocks downgrading from 12.1.3 to lower versions to prevent jailbreaking.
So if your phone seems sluggish after iOS 12.1.3 upgrade and if you wish to downgrade back to a lower version, it seems you simply cannot as Apple has blocked it.
As reported by SoftPedia, Apple has allegedly stopped signing the iOS 12.1.1 and iOS 12.1.2 firmware for all supported iPhones and iPads. With this move, February 5th, 2019 was the last date anyone could downgrade their devices to an older iOS version if they faced any trouble. You cannot downgrade using iTunes or any other means.
This seems to be a major impact on the jailbreaking community. The community completely relies on older iOS firmware which helps them to successfully jailbreak into devices.
Apple seems to limit the spread of older firmware on its iOS devices in an attempt to keep iPhone and iPad users secure.