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There is some lag but I just say what I need to say and within a few seconds it appears on the screen. Even when dictating to my iPhone there is some lag, the BT is just adding a little more.

Siri is a great help for me. It allows me to set reminders, timers, etc with out even pulling out my phone. My only request is to have Siri answer or confirm with audio on my watch.
 
It's response time if you just work on the connection to the network or wifi will be based on your ping
 
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