First response time is the number customers feel most. It is usually measured in business hours when support isn’t staffed around the clock, so a message sent at 5:40 pm on Friday is judged from Monday morning.
Automatic acknowledgements and emoji reactions normally don’t count as a first response. Report FRT as the share of requests answered within target plus a high percentile such as the 90th, rather than an average, which hides the slowest replies. The first response time calculator shows when a reply is due for any working hours.