https://finance.yahoo.com/news/apple-ios-12-preview-apos-211100225.html

Given that the X is the newest, fastest phone in Apple's arsenal, you might not expect to see much of a difference between a unit running iOS 11.4 and one with 12. You'd be mostly right. My iPhone X running the public beta was marginally faster than an iPhone X on iOS 11.4, but don't expect this to be the case all the time. Apple has said that launching apps, loading the keyboard and accessing the share sheet inside apps generally happen faster in iOS 12, and my experience testing these two iPhone Xs side by side bore that out. While it does make the process of interacting with the iPhone X feel slightly smoother, it's not the kind of thing you'd necessarily notice in day-to-day use.

It's also worth noting that although you probably shouldn't use this public beta build as your daily driver, I haven't run into any deal-breaking issues just yet. That doesn't mean you won't, or that there aren't any to be found -- this software is still a ways off from completion, so I'd still only recommend throwing the beta onto a device you don't rely on every day.