I have a Stainless Steel S0 which I got on launch day when it was released. It was slow the day I received it and it is slow today.
However, for my use, which is just notifications + starting workouts + tracking fitness + time keeping, it was fine. As long as I didn't launch any apps (aside from the Workout/Activity app which I keep on the watch face so they launch instantly), it performs just fine. The battery lasts the entire work day for me, including workouts, from 9:00AM from when I put it on to around 12:00AM when I go to sleep. From a 4 year old watch, this is pretty amazing.
Honestly, if Apple didn't EOL the S0 with the watchOS updates this year, I would have eeked one more year out of it.
If you look at this iPhone-iOS trend—
https://twitter.com/OneJaredNewman/status/1040671936660103168—it seems like every two years, the number of major iOS updates supported by each iPhone increases. It makes sense that this would be true for the watch as well if they keep making the watch faster, thinner and make the battery last longer. So I'm hoping to get at least 5-6 years use out of the S4.
...anyway, that's how I justified me spending almost $1000 on the watch this year.