watchOS 9 performs well on my Series 5 Watch. I’m not experiencing any issues at this point and I don’t foresee any either.
My only complaint is that they have moved Workout Views from the phone to the watch and, while it offers quite a bit more customization than before, it is more complicated to make changes.
Man, I hate that you can't do this in the Watch app on the phone anymore. It's a lot more complicated this way. I like that you can add some of these extra screens, and particularly like that there is way to check out the progress on rings from the workout screen without exiting the workout, but it's not the easiest way to get this done.
I'm still recovering from that ankle injury in June and I think it'll be a while before I do a running workout, so I won't be able to share what I set up yet.