A few days ago I wrote about how I was gradually growing less satisfied with the responsiveness of my Apple Watch with the added bloat of newer systems, especially to the Hey Siri command, which besides actually looking at the watch has been my most commonly used interaction with it. Today I stumbled upon a setting that made a huge difference in Siri responsiveness: For some reason I held my phone in my hand as I summoned Siri to trigger a timer for me, and I realized that the phone rather than the watch responded to the command. I had activated Hey Siri on the phone a while ago from curiosity and never turned it off.
This time I did turn off Hey Siri functionality on the phone, and sure enough, response time to the command on the watch dropped to become almost instantaneous. It still takes a while for the command uttered to be processed by the watch, but it now has restored my trust in it happening in most cases, which has returned my Apple Watch to its status of “significantly more useful than a regular watch”.
As to what caused the issue, my current hypothesis is that the faster CPU in the iPhone 6S realized “Hey Siri” had been said first, then the watch chimed in and tried to claim the action for what had been said since it was the active device at the time, and this whole negotiation process was what made Siri on the Watch unbearably slow to use.