Hello.
It seems like watchOS 5 comes to the Apple Watch Series 1 with all new features, what are:
- Walki-Talkie
- new Workouts like yoga and hiking
- competitions with friends in Activity
- automatic workout detection
- Siri watch face is about to get smarter with 3rd party apps
- Siri is also getting Shortcuts just like on iOS 12
- you don't have to say "Hey, Siri" anymore, just raise your wrist
- podcasts app
- webkit (you can view links to the web sites)
I haven't tested the beta on my Apple Watch, cause according to the reviews, it was pretty baggy.
So if it slows down the Series 1, we will find out on the release later in September.
You can find out more about watchOS5 more on the Apple's website
watchOS 5 Preview - Apple