You must have a better watch than mine. Siri is massively unreliable and often very slow to input what I said. I try to reply to a message, and it doesn't work. I tried to set a timer a day ago, and it just sat there forever. I finally end up giving up and using my phone.The watch is plenty fast as it is. It could be faster but notifications, Siri and the built in apps all work perfectly.
There is independent wifi. While connected via Bluetooth, turn off wifi on iPhone. Then turn it back on. Then turn Bluetooth off. Your watch will stay connected to wifi.
I mainly use the watch for notifications and it does the job quite well. Battery life could be improved and I wouldn't complain but I don't want a camera and if I want to make a phone call I use my phone! It's already thin and doesn't get in the way.