VoiceOver Performance on Apple Watch Series

Well, I bought a Series 1 last year and at the moment, I find it somewhat snappy. Well, when swiping, it responds well enough, no major lag but then again, I haven't installed Watch OS 4 yet. Even after seeing last week's keynote, I don't find the need to upgrade to a Series 3, going to wait another year or two. I am hoping that once I put Watch OS 4, the watch will still run smoothly as it does now.

I just got a Series 3 (WiFi model) today so I've not done that much other than set it up. I have had the original Apple watch since it was released. The original was never fast and I've experienced more lag over the OS releases. My most annoying was at times it would ignore my taps such as making me enter my PIN twice, or not responding when the Workout app was running. My initial impression of Series 3 is the performance is much better as one would expect. It wakes up more quickly and so far responds to my taps the first time. As Apple mentioned Siri also speaks though VoiceOver users already got feedback so I am not sure you would notice a huge change. Other than on my original model it was difficult to get Siri to do anything at all especially in WatchOS 3, it did improve slightly in WatchOS 4.
So yes I believe you will notice an improvement. If it is worth the cost depends on your personal use case, and so forth, that story has not changed. Hope this helps.