Daniel Jalkut on Siri's new behavior:
Apple “broke” the haptic feedback associated with invoking Siri, by “fixing” the problem that there had ever been any latency before. Have an iPhone 6s or 6s Plus? Go ahead, I dare you: hold down the home button and start talking to Siri. You will not escape its attention. It’s ready to go when you are, so it would be obnoxious of it to impose any contrived delay or to give taptic feedback that is uncalled for. Siri has become a more perfect assistant, and we have to change our habits to accommodate this.
Great little detail of Siri that I didn't notice until today. Siri seems to agree, too.