You Can Downgrade Your iPhone X
If you can't stand iOS 12.3.1's performance you can try downgrading to iOS 12.3.
Apple is signing on iOS 12.3 which means you can use a loophole to drop your iPhone X back down to the previous version of iOS 12.
Apple's closed off the downgrade paths to iOS 12.2, iOS 12.1.4, iOS 12.1.3, iOS 12.1.2, iOS 12.1.1, iOS 12.1, iOS 12.0.1, iOS 12.0 and iOS 11 which means you can't move to any of them.
If you're interested in the downgrade process, take a look at our downgrade guide.