I have been finding myself using Siri more on my Watch then I ever did on my phone. Raising my wrist and saying "Hey Siri" usually works pretty well and I do it a lot around my house and office.
I noticed Siri wasn't as responsive when I was out and about, and initially thought it might have to do with background noise, wind, etc. Tonight I happened to try it while my phone was out. I noticed the phone had a WiFi connection to Comcast's xfinitywifi service, but only one bar. The iPhone hangs onto any WiFi connection (even a lousy one) before switching back to much faster LTE. I killed the WiFi and Siri was immediately back to her quick, helpful self.
Comcast's WiFi is pervasive, but often is actually delivered as a side effect from private homes and businesses, so it can be very inconsistent. It's handy in a pinch but I will leave it off my iPhone list of WiFi networks since it is effectively slowing down my Siri requests with bad connections.