First response time is the time between a customer sending their first message and your team sending its first reply, not counting automatic acknowledgements.
To calculate it, take the time of your first human-written or approved reply, subtract the time the customer's message arrived, and average that across conversations. Decide whether you measure in business hours or calendar hours and stick to it. A message sent at 9pm on Friday and answered at 9am on Monday is 60 calendar hours but close to zero business hours.
An automatic "we got your message" email does not count. Customers can tell the difference, and including it makes the number look better than the experience.
For a small shop, the fastest win is having a good draft ready when you open the inbox. Hobson drafts a reply to every conversation with the order and tracking looked up, so approving it takes seconds. See AI drafting.