Animations everywhere are smoother, e.g. bringing up the app switcher, scrolling through the app switcher, swiping up control center and swiping down Notification Center, force touching watch face, and so on.
Also the Workout app is now a bit faster to resume (it still pauses for a couple of seconds, but it’s definitely faster than it used to be).
If they can keep this smoothness with watchOS 5 I’ll be really happy, but usually performance is bad in the first version and a few point releases. It’s be nice if that’s different this year given the focus on performance in iOS 12, but the watchOS beta bugginess isn’t filling me with confidence.